Imitate readthedocs

Imitate GitHub markdown rendering

Fixes diagrams URL in DOM

Conflicts:
	doc/stream.md
	doc/tutorial.md

But some examples are to be filled in.

Integrate Markdown pages into Doxygen documentation

To avoid the {#mainpage} tag to be rendered on GitHub, explicitly add a
`\mainpage` section to `rapidjson.h`, referring to `readme.md`.

Rename nested typedef to avoid confusion with JSON Type enum.