docs: remove unused mentoring mailing list reference

docs: remove unused mentoring mailing list reference

The git-mentoring group was initially created to help newcomers
with their development itches. However, in practice,
most of their questions were already being addressed
directly on the mailing list, and contributors consistently
received helpful responses there.

Remove the mentoring group details from the Documentation.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>