External reverse-proxy apache or nginx configuration

Contribution to solve https://gitlab.com/gitlab-org/gitlab-ce/issues/35079

See the general Documentation guidelines: https://docs.gitlab.com/ee/development/documentation/.

What does this MR do?

External reverse-proxy apache or nginx configuration

Changing the header title?

I add a sub-tile "Custom nginx or apache config as reverse proxy to gitlab"

Moving docs to a new location?

No

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/35079.