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

The error messages in ParseNumber used `is.Tell` to report the
position of the number parsing error.  Depending on the copy
optimization of the current stream, this can lead to different
behaviour (beginning of number vs. position of error).

 * Delegate constant string construction to SetStringRaw
 * Delegate "const Ch*" overloads to GenericValue variants
   of operator[], FindMember and RemoveMember
 * Remove repeated template arguments in nested struct Array

(cherry-picked from ca9b0332d)

Instead of hard-coding the value 0 for the parseFlags in the
various parsing overloads, explicitly use kParseDefaultFlags
to provide more self-documenting code.