doc: switch from .md to .dox files and impose some general structure

This is an attempt of breaking up the documentation, currently spread across
several pages. We're left with a few high-level topics with various things
grouped underneath those.

Further refinement is necessary, but this can now be done in incremental steps
over massive reworks.

doc/pipewire-daemon.dox

@@ -0,0 +1,43 @@
+/** \page page_daemon PipeWire Daemon
+
+
+\section sec_logging Logging
+
+The `PIPEWIRE_DEBUG` environment variable can be used to enable
+more debugging. The format is:
+
+`<level>[<category>;...]`
+
+- `<level>` specifies the log level:
+  + `0`: no logging is enabled
+  + `1`: Error logging is enabled
+  + `2`: Warnings are enabled
+  + `3`: Informational messages are enabled
+  + `4`: Debug messages are enabled
+  + `5`: Trace messages are enabled. These messages can be logged
+     	from the realtime threads.
+
+- `<category>`:  Specifies a string category to enable. Many categories
+     	  can be separated by commas. Current categories are:
+  + `connection`: to log connection messages
+
+
+\subsection sec_errors Error reporting
+
+Functions return either NULL with errno set or a negative int error
+code when an error occurs. Error codes are used from the SPA plugin
+library on which PipeWire is built.
+
+Some functions might return asynchronously. The error code for such
+functions is positive and SPA_RESULT_IS_ASYNC() will return true.
+SPA_RESULT_ASYNC_SEQ() can be used to get the unique sequence number
+associated with the async operation.
+
+The object returning the async result code will have some way to
+signal the completion of the async operation (with, for example, a
+callback). The sequence number can be used to see which operation
+completed.
+
+
+
+*/