Add man page generation via clap_mangen

Add --gen-man-page flag that renders a man page to stdout. Build-time
generation is also supported via the MAN_PATH env var, mirroring the
existing COMPLETION_PATH pattern.

Cargo.toml

@@ -38,6 +38,7 @@ anyhow        = "1.0"
 byte-unit     = "5.2"
 clap          = {version = "4.4", features = ["derive"]}
 clap_complete = "4.4"
+clap_mangen   = "0.2"
 console       = "0.16.2"
 chrono        = {version = "0.4.44", default-features = false, features = ["clock"]}
 directories   = "6.0.0"
@@ -57,6 +58,7 @@ unicode-width = "0.2"
 anyhow        = "1.0"
 clap          = {version = "4.4", features = ["derive"]}
 clap_complete = "4.4"
+clap_mangen   = "0.2"
 
 [target.'cfg(any(target_os = "linux", target_os = "android"))'.dependencies]
 pager         = "0.16.1"