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/spa/buffer.md

@@ -1,3 +1,5 @@
+# SPA Buffers
+
 > What is the array of `spa_data` in `spa_buffer`?
 
 A buffer contains metadata and data. There can be many metadata items (headers, color info, cursor position, etc) in the buffer. The metadata items are stored in the metas array. In the same way, the buffer can contain multiple data blocks in the datas array. Each data block is, for example, a video plane or an audio channel. There are `n_datas` of those blocks.

doc/spa/index.md

@@ -1,4 +1,9 @@
 # SPA (Simple Plugin API)
 
+SPA (Simple Plugin API) is an extensible API to implement all kinds of
+plugins. It is inspired by many other plugin APIs, mostly LV2 and
+GStreamer.
+
 * SPA [Design](design.md)
 * [Data format](pod.md)
+* SPA [Buffers](buffer.md)

doc/spa/pod.md

@@ -1,4 +1,4 @@
-# POD
+# SPA POD
 
 POD (plain old data) is a sort of data container. It is comparable to
 DBus Variant or LV2 Atom.