DOC: Split converters/index.rst into separate files

src/doc/concepts.rst

src/doc/converters/cfood_definition.rst

src/doc/converters/custom_converters.rst

src/doc/converters/further_converters.rst

src/doc/converters/index.rst

src/doc/converters/standard_converters.rst

src/doc/converters/transform_functions.rst