Split gitlab-sshd docs into its own document

What does this MR do and why?

Rework fast SSH key lookup document to make it clear that this feature can be used with gitlab-sshd or OpenSSH.

This is a follow-up for !111383 (merged).

Review app links:

• https://main-ee-111505.docs.gitlab-review.app/ee/administration/operations/gitlab-sshd.html
• https://main-ee-111505.docs.gitlab-review.app/ee/administration/operations/fast_ssh_key_lookup.html#set-up-fast-lookup

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.