Add documentation for fixing SELinux contexts

What does this MR do?

This MR added documentation for Omnibus Gitlab installation with SElinux enabled and custom Gitlab directory with symbolic links.

Related issues

On Omnibus Gitlab installation with SElinux enabled, when we install gitlab on custom path with symbolic link to /etc/gitlab/ and /var/opt/gitlab/, users cannot connect to Gitlab with ssh key.

There will be error like this after user send the public key:

debug3: receive packet: type 51

To fix this, Gitlab admin need to reconfigure the SELinux with the command on this MR.

Please see: #3828 (comment 106326334)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened