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.
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