doc: reorganize files

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

doc/dox/api/spa-index.dox

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+/** \page page_spa SPA (Simple Plugin API)
+
+\ref 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 provides two parts:
+
+- A header-only API with no external dependencies.
+- A set of support libraries ("plugins") for commonly used functionality.
+
+The usual approach is that PipeWire and PipeWire clients can use the
+header-only functions to interact with the plugins. Those plugins are
+usually loaded at runtime (through `dlopen(3)`).
+
+
+# Motivation
+
+SPA was designed with the following goals in mind:
+
+- No dependencies, SPA is shipped as a set of header files that have no dependencies except for the standard C library.
+- Very efficient both in space and in time.
+- Very configurable and usable in many different environments. All aspects
+  of the plugin environment can be configured and changed, like logging,
+  poll loops, system calls, etc.
+- Consistent API.
+- Extensible; new API can be added with minimal effort, existing API can be updated and versioned.
+
+The original user of SPA is PipeWire, which uses SPA to implement the
+low-level multimedia processing plugins, device detection, mainloops, CPU
+detection, logging, among other things. SPA however can be used outside
+of PipeWire with minimal problems.
+
+
+# The SPA Header-Only API
+
+A very simple example on how SPA headers work are the \ref spa_utils, a set
+of utilities commonly required by C projects. SPA functions use the `spa_`
+namespace and are easy to identify.
+
+\code
+/* cc $(pkg-config --cflags libspa-0.2) -o spa-test spa-test.c */
+
+#include <stdint.h>
+#include <spa/utils/string.h>
+
+int main(int argc, char **argv) {
+	uint32_t val;
+
+	if (spa_atoi32(argv[1], &val, 16))
+		printf("argv[1] is hex %#x\n", val);
+	else
+		printf("argv[1] is not a hex number\n");
+
+	return 0;
+}
+\endcode
+
+
+# SPA Plugins
+
+SPA plugins are shared libraries (`.so` files) that can be loaded at
+runtime. Each library provides one or more "factories", each of which may
+implement several "interfaces". Code that uses SPA plugins then uses those
+interfaces (through SPA header files) to interact with the plugin.
+
+For example, the PipeWire daemon can load the normal `printf`-based logger
+or a systemd journal-based logger. Both of those provide the \ref spa_log
+interface and once instantiated, PipeWire no longer has to differentiate
+between the two logging facilities.
+
+Please see \ref page_spa_plugins for the details on how to use SPA plugins.
+
+
+# Further details
+
+- \ref api_spa
+- \subpage page_spa_design
+- \subpage page_spa_plugins
+- \subpage page_spa_pod
+- \subpage page_spa_buffer
+
+
+\addtogroup api_spa
+
+See: \ref page_spa, \ref page_spa_design
+
+*/