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.