MLECO-6237: Add missing documentation

* Add missing docstring

* Rename the following readme's to resolve:
  Doxygen warning: found more than one mainpage
  1) src/cpp/config/README.md to intro.md
  2) src/cpp/frameworks/README.md to intro.md

Signed-off-by: Conor Kennedy <conor.kennedy@arm.com>
Change-Id: I7d1ebed3eef575a547ab7a5bc0dac0454180122b