QtFRED help preferences (#7928)

* remove the unregistered duplicate qtFRED Preferences help page

Two Preferences pages exist, both added in #7380:
general/PreferencesDialog.html, which qtfred.qhp registers as its section,
keyword and file, and dialogs/PreferencesDialog.html, which nothing registers.

An unregistered page is not compiled into qtfred_help.qch, so the second page has
never been reachable in the built help. It is also out of date: its General table
lists "Move ships with arrow keys", "Check for errors on save", "Show SEXP help
tooltips" and a Dark mode checkbox, none of which are on that tab, and its
Keybindings heading describes what the dialog calls Controls. The registered page
documents the dialog as it is, including autosave, the grid and remapping
controls.

Nothing links to the duplicate, so removing it changes no link and leaves the
built help identical.

* fix the Mission Specs link on the qtFRED Music Player help page

The link pointed at MissionSpecDialog.html; the page is MissionSpecsDialog.html,
so the link went nowhere in the built help.

* drop the help index entry for a Form Wing page that does not exist

index.html listed Form Wing under Objects and linked dialogs/FormWingDialog.html,
which is not in the tree and is not registered in qtfred.qhp, so the entry was a
dead link. The dialog is a single text box for the wing's name, and forming a
wing is already covered in the quickstart, the fundamentals page and the toolbar
reference, so no page is needed.

* register the Custom Data concept help page

concepts/custom-data.html was in the tree and linked from the Mission Specs
dialog page, but was not listed in qtfred.qhp, so it was not compiled into
qtfred_help.qch and the link to it went nowhere. Adds it to the Concepts section
of the table of contents, the keyword index and the file list.

qtfred/help-src/doc/dialogs/MusicPlayerDialog.html

@@ -11,7 +11,7 @@
 <p>Opens via <strong>View &rsaquo; Music Player</strong>.</p>
 <p>Plays back music files from the game's music folder while you work, so you can
 audition tracks without launching FreeSpace Open. Useful when choosing tracks for
-<a href="MissionSpecDialog.html">Mission Specs</a>,
+<a href="MissionSpecsDialog.html">Mission Specs</a>,
 <a href="BriefingEditorDialog.html">Briefing</a>,
 <a href="DebriefingDialog.html">Debriefing</a>, or
 <a href="CommandBriefingDialog.html">Command Briefing</a>.</p>

qtfred/help-src/doc/dialogs/PreferencesDialog.html

@@ -1,43 +0,0 @@
-<!DOCTYPE html>
-<html lang="en">
-<head>
-    <meta charset="UTF-8"/>
-    <title>Preferences - QtFRED Help</title>
-    <link rel="stylesheet" type="text/css" href="../css/help.css"/>
-</head>
-<body>
-<nav class="breadcrumb"><a href="../index.html">QtFRED Help</a> &rsaquo; Tools &rsaquo; Preferences</nav>
-<h1>Preferences</h1>
-<p>Opens via <strong>File &rsaquo; Preferences</strong>.</p>
-<p>Controls QtFRED editor settings. Changes take effect immediately; there is no
-Apply button.</p>
-
-<h2>General</h2>
-<table>
-    <tr><th>Setting</th><th>Description</th></tr>
-    <tr><td>Move ships with arrow keys</td><td>When enabled, the arrow keys nudge
-        selected objects in the viewport instead of scrolling the view.</td></tr>
-    <tr><td>Check for errors on save</td><td>Runs the built-in mission error checker
-        automatically each time the mission is saved.</td></tr>
-    <tr><td>Show SEXP help tooltips</td><td>Displays documentation tooltips when
-        hovering over SEXP nodes in the condition and action trees.</td></tr>
-    <tr><td>Dark mode</td><td>Switches the editor UI to a dark color
-        theme.</td></tr>
-</table>
-
-<h2>Grid</h2>
-<table>
-    <tr><th>Setting</th><th>Description</th></tr>
-    <tr><td>Grid plane</td><td>Which plane the editor grid lies on:
-        <kbd>XZ</kbd> (horizontal), <kbd>XY</kbd>, or <kbd>YZ</kbd>.</td></tr>
-    <tr><td>Grid center</td><td>The world-space coordinates of the grid's center
-        point.</td></tr>
-    <tr><td>Reset grid</td><td>Resets the grid plane and center to defaults.</td></tr>
-</table>
-
-<h2>Keybindings</h2>
-<p>Lists every editor action alongside its current keyboard shortcut. Click a shortcut
-field and press a new key combination to rebind it. Conflicts are highlighted.
-Bindings are saved per-user and persist across sessions.</p>
-</body>
-</html>

qtfred/help-src/doc/index.html

@@ -68,7 +68,6 @@ covers the basics of mission creation, the editor dialogs, and advanced topics.<
 
 <h3>Objects</h3>
 <ul>
-    <li><a href="dialogs/FormWingDialog.html">Form Wing</a></li>
     <li><a href="dialogs/JumpNodeEditorDialog.html">Jump Node Editor</a></li>
     <li><a href="dialogs/ObjectOrientEditorDialog.html">Object Orientation</a></li>
     <li><a href="dialogs/PropEditorDialog.html">Prop Editor</a></li>

qtfred/help-src/doc/qtfred.qhp

@@ -10,6 +10,7 @@
                 <section title="Concepts">
                     <section title="AI"              ref="concepts/ai.html"/>
                     <section title="Campaign Flow"   ref="concepts/campaign-flow.html"/>
+                    <section title="Custom Data"     ref="concepts/custom-data.html"/>
                     <section title="IFF &amp; Teams" ref="concepts/iff.html"/>
                     <section title="Messages"        ref="concepts/messages.html"/>
                     <section title="SEXPs"           ref="concepts/sexps.html"/>
@@ -93,6 +94,7 @@
             <keyword name="AI"                      id="concept.ai"                 ref="concepts/ai.html"/>
             <keyword name="Messages"                id="concept.messages"           ref="concepts/messages.html"/>
             <keyword name="Campaign Flow"           id="concept.campaignflow"       ref="concepts/campaign-flow.html"/>
+            <keyword name="Custom Data Concepts"    id="concept.customdata"         ref="concepts/custom-data.html"/>
             <keyword name="Viewport"                id="concept.viewport"           ref="general/Viewport.html"/>
             <keyword name="Toolbars"                id="concept.toolbar"            ref="general/Toolbars.html"/>
             <keyword name="Custom Data"             id="CustomDataDialog"           ref="dialogs/CustomDataDialog.html"/>
@@ -158,6 +160,7 @@
             <file>concepts/ai.html</file>
             <file>concepts/messages.html</file>
             <file>concepts/campaign-flow.html</file>
+            <file>concepts/custom-data.html</file>
             <file>dialogs/CustomDataDialog.html</file>
             <file>css/help.css</file>
             <file>dialogs/AsteroidEditorDialog.html</file>