GitLab Shell: make extended configuration of sshd_config

See merge request gitlab-org/charts/gitlab!3581



Merged-by: Mitchell Nielsen <mnielsen@gitlab.com>
Approved-by: Dmytro Makovey <dmakovey@gitlab.com>
Approved-by: Mitchell Nielsen <mnielsen@gitlab.com>
Co-authored-by: Jason Plum <jplum@gitlab.com>

(cherry picked from commit 3528f292)

ba4d7f44 GitLab Shell: make extended configuration of sshd_config
eb74e408 GitLab Shell: examples, refine and add sshd_config.d mount
985cbeeb GitLab Shell: add documentation for extended sshd_config
4867bed1 GitLab Shell: align examples to docs
99ef9012 Markdown linting touch-up