HPCC-9593 Added documentation pertaining to cmake structure

Cmake structure outline and brief documentation 1st draft.

Signed-off-by: Philip Schwartz <philip.schwartz@lexisnexis.com>

Reworked a bit prior to pulling.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-9593 Added documentation pertaining to cmake structure

Reworked into standard filenames and add rst formatting.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4599 from richardkchapman/cmake_docs

Cmake docs

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>