man: Use asciidoc for main man page as well

We already use asciidoc for the gnome-extensions man page, while
the main man page is generated from docbook XML. The former is a
much friendlier source format, so use it for both man pages.

Hopefully the plain text format encourages updates, to prevent the
page from getting as badly out of sync again as it is currently.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1553>

man/meson.build

@@ -1,19 +1,7 @@
 custom_target('man page',
-  input: 'gnome-shell.xml',
+  input: ['gnome-shell.txt', 'stylesheet.xsl'],
   output: 'gnome-shell.1',
-  command: [
-    xsltproc,
-    '-o', '@OUTPUT@',
-    '--nonet',
-    '--stringparam', 'man.output.quietly', '1',
-    '--stringparam', 'funcsynopsis.style', 'ansi',
-    '--stringparam', 'man.th.extra1.suppress', '1',
-    '--stringparam', 'man.authors.section.enabled', '0',
-    '--stringparam', 'man.copyright.section.enabled', '0',
-    'http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl',
-    '@INPUT@'
-  ],
-  build_by_default: true,
+  command: [a2x, '-D', '@OUTDIR@', '--xsl-file', '@INPUT1@', '-f' ,'manpage', '@INPUT0@'],
   install_dir: mandir + '/man1',
   install: true
 )