bpf, docs: Fix ordering of bpf documentation

This commit fixes the display of the BPF documentation in the sidebar
when rendered as HTML.

Before this patch, the sidebar would render as follows for some
sections:

| BPF Documentation
  |- BPF Type Format (BTF)
    |- BPF Type Format (BTF)

This was due to creating a heading in index.rst followed by
a sphinx toctree, where the file referenced carries the same
title as the section heading.

To fix this I applied a pattern that has been established in other
subfolders of Documentation:

1. Re-wrote index.rst to have a single toctree
2. Split the sections out in to their own files

Additionally maps.rst and programs.rst make use of a glob pattern to
include map_* or prog_* rst files in their toctree, meaning future map
or program type documentation will be automatically included.

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1a1eed800e7b9dc13b458de113a489641519b0cc.1636749493.git.dave@dtucker.co.uk

docs/index.rst

@@ -3,8 +3,6 @@
 libbpf
 ======
 
-For API documentation see the `versioned API documentation site <https://libbpf.readthedocs.io/en/latest/api.html>`_.
-
 .. toctree::
    :maxdepth: 1
 
@@ -14,6 +12,8 @@ For API documentation see the `versioned API documentation site <https://libbpf.
 This is documentation for libbpf, a userspace library for loading and
 interacting with bpf programs.
 
+For API documentation see the `versioned API documentation site <https://libbpf.readthedocs.io/en/latest/api.html>`_.
+
 All general BPF questions, including kernel functionality, libbpf APIs and
 their application, should be sent to bpf@vger.kernel.org mailing list.
 You can `subscribe <http://vger.kernel.org/vger-lists.html#bpf>`_ to the