Pages configuration switch API/disc source

Implement switch discussed in gitlab-pages#351 (closed)

• Add /internal/pages/status endpoint !28743 (merged)
• [-] Add -disable-gitlab-api-config-source command-line option to Pages gitlab-pages!264 (closed)
• [-] Add related changes to omnibus omnibus-gitlab!4057 (closed)
• [-] Documentation update !29103 (closed)

Follow up issues

• Add domain-source configuration #217912 (closed)
• Update omnibus to use domain-source=auto omnibus-gitlab#5350 (closed)
• Update omnibus to use domain-source=api omnibus-gitlab#5351 (closed)
• Deprecate domain-source #217913 (closed)
• Remove domain-source and all of disk source related code gitlab-pages#382 (closed)