[3.12] Docs: rework dbm introduction (GH-114551) (#114590)

[3.12] Docs: rework dbm introduction (GH-114551) (#114590)

- add refs to other parts of the docs (dict, bytes, etc.)
- clarify whichdb() return value by using list markup
- silence refs to example or generic submodule methods (keys, get, etc.)

(cherry picked from commit 65cf5dce11a38e327b9b0abfca279d650452b34f)

Co-authored-by: Erlend E. Aasland <erlend@python.org>