Move old misplaced docs out of root directories

We have several old docs that are in root directories by themselves, and they should be moved to the appropriate places:

• system_hooks/system_hooks.md -> administration/system_hooks.md - !83169 (merged)
• public_access/public_access.md -> user/public_access.md - !83172 (merged)
• ssh/ssh.md -> user/ssh.md - !83173 (merged)
• tools/email.md -> user/admin_area/email_from_gitlab.md - !83165 (merged)

After moving them all, we need to:

• Update the nav to point to the new locations. - gitlab-docs!2513 (merged)