docs: kbase: internals: Make 'overview' page useful and link to it

docs: kbase: internals: Make 'overview' page useful and link to it

While the content is slightly outdated it's still a good primer on how
an API call traverses through the client library and to the remote
driver.

To make the page useful, this commit:
 - removes the paragraphs which were intended to serve as a directory
   page for the 'internals' subdirectory
 - adds a note saying that some facts might not be up to date
 - adds linking to this page from the kbase directory page
 - adds more monospace formatting around function names

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