docs: Convert 'internals' to RST and move it to 'kbase/internal/overview.rst'

docs: Convert 'internals' to RST and move it to 'kbase/internal/overview.rst'

Note that this document was not referenced from any top level page. This
patch does a straight conversion and leaves it unreferenced.

Next patch will then modify it to serve as an overview (hence the new
name) of how an API call happens.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>