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/index.dox

@@ -0,0 +1,35 @@
+/** \mainpage PipeWire
+
+PipeWire is low-level multimedia framework that provides:
+  - Graph based processing
+  - Support for out-of-process processing graphs with minimal overhead
+  - Flexible and extensible media format negotiation and buffer allocation
+  - Hard real-time capable plugins
+  - Very low-latency for both audio and video processing
+
+See \ref page_overview for an overview of PipeWire and \ref page_design
+for the design principles guiding PipeWire.
+
+### Components
+
+PipeWire ships with the following components:
+
+- a \ref page_daemon that implements the IPC and graph processing
+- an example \ref page_session_manager that manages objects in the \ref page_daemon
+- a set of \ref page_tools to introspect and use the \ref page_daemon
+- a \ref page_library to develop PipeWire applications and plugins (\ref
+  page_tutorial "tutorial").
+- the \ref page_spa used by both the \ref page_daemon and in the \ref
+  page_library.
+
+### API Documentation
+
+See \ref page_api
+
+
+### Resources
+
+* [LAC 2020 Paper](https://lac2020.sciencesconf.org/307881/document)
+
+
+*/