doc: add the various markdown prose docs to the doxygen output

Note that the order of the includes matters - that's how doxygen will sort
them. There is no specific structure other than the include order - one reason
why the headers are being changed. Without polluting the markdown files with
doxygen commands we cannot use \subpage, so all files convert to a regular
\page and are listed as flat hierarchy in the sidebar (and in Related Pages).

Changing the headers at least provides some visual grouping with comon
prefixes.

doc/architecture.md

@@ -1,9 +1,9 @@
-# Architecture
+# PipeWire Architecture
 
 There are 2 main components that make up the PipeWire library:
 
- 1) An implementation of a graph based media processing engine.
- 2) An asynchronous IPC mechanism to manipulate and introspect
+1. An implementation of a graph based media processing engine.
+2. An asynchronous IPC mechanism to manipulate and introspect
     a graph in another process.
 
 There is usually a daemon that implements the global graph and