doc: reorganize files

Separate various autogen files from the documentation .dox files.
Rename .dox files to match the intended tree structure.

doc/spa-design.dox

@@ -1,35 +0,0 @@
-/** \page page_spa_design SPA Design
-
-# Conventions
-
-## Types
-
-Types are generally divided into two categories:
-
-- String types: They identify interfaces and highlevel object types.
-- Integer types: These are enumerations used in the parts where high
-                 performance/ease of use/low space overhead is needed.
-
-The SPA type is system is statis and very simple but still allows you
-to make and introspect complex object type hierarchies.
-
-See the type system docs for more info.
-
-## Error Codes
-
-SPA uses negative integers as errno style error codes. Functions that return an
-int result code generated an error when < 0. `spa_strerror()` can be used to
-get a string representation of the error code.
-
-SPA also has a way to encode asynchronous results. This is done by setting a
-high bit (bit 30, the `ASYNC_BIT`) in the result code and a sequence number
-in the lower bits. This result is normally identified as a positive success
-result code and the sequence number can later be matched to the completion
-event.
-
-## Useful Macros
-
-SPA comes with some useful macros defined in `<spa/utils/defs.h>` and a
-number of utility functions, see \ref spa_utils.
-
-*/