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.

Although not yet complete, the rapidjson.h header contains most of
the customisation macro definitions.  These should be documented via
Doxygen as well.  This requires adding file-level documentation.

Some parts are excluded via `@cond` to avoid cluttering the documentation
with internals too much.

Secondly, a brief description is added to the 'rapidjson' namespace to
include the namespace-level elements (enums, typedefs, functions) to
the generated documentation.

C-style cast
Add function const to MemoryPoolAllocator::Capacity() and Size()

Added unit test for parsing EncodedStream to Document.

Safer implementation of Swap()

Also added documentation, unit tests for swapping Document/Value,
Document/Document.

Miscellaneous cleanups