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 v8.108.15 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. Instead we removed Functionbeat from kbn-doc-links via kbn-doc-links: remove functionbeat kibana#198902
• 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

[lucabelluccini]

As reported from users - the fact we update docs and we publish statements such as Version 8.15.2 supports deploying Functionbeat as an AWS Lambda servic.... is adding confusion.

[kilfoyle]

Thanks @lucabelluccini! I've followed up in the Integrations channel in Slack to see if I can get some traction on this issue.
@nimarezainia just FYI.

[AndersonQ]

@nimarezainia, elastic/beats#40745 is about removing it only for 9.0. I was talking to @kilfoyle over slack and we were unsure if the docs should be kept for 8.16 and 8.x or not.

[kilfoyle]

Functionbeat docs are now frozen at 8.15 and moved to the "Legacy docs" section of the main docs page, via elastic/docs#2895

Thanks so much for your help with this @AndersonQ!