Deprecate Functionbeat docs

[kilfoyle]

Functionbeat reaches end of support on Oct.18, 2023. The docs are being updated to indicate that via elastic/beats#36871

We need to deprecate the Functionbeat docs:

• Freeze the builds at v8.10 and move the guide to the "Legacy documentation" part of the docs landing page. Freeze published Functionbeat docs at 8.10 docs#2814 Freeze Functionbeat docs at 8.15 docs#2895
• Fix broken links caused by freezing. ← ON HOLD. Links from UI are being fixed.
• Update the [main Functionbeat docs page].(https://www.elastic.co/guide/en/beats/functionbeat/current/functionbeat-overview.html) with a notice and a link to the Elastic Serverless Forwarder Guide. Mark Functionbeat documentation as deprecated; point to ESF docs beats#37021

Rel: https://github.com/elastic/ingest-dev/issues/1251