doc: move the overview page out to its own page

This is a good first (top) page to have in the docs, let's move it out
from the pipewire heading.

This doesn't change any of the links and the renaming isn't required
(subpage controls whether it's a top-level item and the meson.build
 order decides its spot in he list). Still, better to have the filenames
represent the layout.

doc/meson.build

@@ -51,8 +51,8 @@ endif
 # Note: order here is how doxygen will expose the pages in the sidebar
 extra_docs = [
   'index.dox',
+  'overview.dox',
   'pipewire.dox',
-  'pipewire-overview.dox',
   'pipewire-design.dox',
   'pipewire-access.dox',
   'pipewire-midi.dox',

doc/pipewire.dox

@@ -2,7 +2,6 @@
 
 ## Internals
 
-- \subpage page_overview
 - \subpage page_design
 - \subpage page_audio
 - \subpage page_access