Doc: Add additional information to the contributing page.

Doc: Add additional information to the contributing page.

Updates for new users related to PR's.
Although some of this info is in other places, a new user should
not need to look at 'commiter' related docs.

Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
MergeDate: Fri Mar 13 08:30:10 2026
(Merged from https://github.com/openssl/openssl/pull/30336)