docs: Port the man pages from DocBook to reStructuredText

So they are consistent with the way we’re building man pages in other
projects, and because some people are allergic to XML.

This changes the build-time dependencies from `xsltproc` to `rst2man`,
and also takes the opportunity to change the `-Dman` Meson option from a
boolean to a feature (so you should use `-Dman-pages={enabled,disabled}`
now, rather than `-Dman={true,false}`).

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/gio/meson.build

@@ -1,12 +1,17 @@
-if get_option('man')
+if get_option('man-pages').enabled()
   manpages = ['gapplication', 'gio-querymodules', 'glib-compile-schemas',
               'glib-compile-resources', 'gsettings', 'gresource', 'gdbus',
               'gio', 'gdbus-codegen']
   foreach page : manpages
     custom_target(page + '-man',
-      input: page + '.xml',
+      input: page + '.rst',
       output: page + '.1',
-      command: xsltproc_command,
+      command: [
+        rst2man,
+        rst2man_flags,
+        '@INPUT@',
+      ],
+      capture: true,
       install: true,
       install_dir: man1_dir,
       install_tag: 'doc',