Build HTML versions of the man pages

And install them in the same directory as the API reference, so it's
possible to link them.

Fixes: #2896

meson.build

@@ -2648,9 +2648,11 @@ if rst2man.found()
   rst2man_flags = [
     '--syntax-highlight=none',
   ]
-  man1_dir = join_paths(glib_prefix, get_option('mandir'), 'man1')
+  man1_dir = glib_prefix / get_option('mandir') / 'man1'
 endif
 
+rst2html5 = find_program('rst2html5', 'rst2html5.py', required: get_option('documentation'))
+
 gnome = import('gnome')
 subdir('docs/reference')