[Docs] - Document manual rollback feature

[pchila]

Document manual rollback feature.

Quick points to be fleshed out (Refer to How to test this PR sections for details)

• Availability:

  • 9.3.0 onwards (for both the agent starting the upgrade and the version we are upgrading to)
  • available for both standalone PR1 PR2 and fleet managed PR

• Configuration:

  • new config agent.upgrade.rollback.window settings (needs to be >0 to activate the feature, new default value when feature is complete should be 7 days, see dedicated task)

• Refer to the PRs above for standalone and fleet managed flows

[nimarezainia]

@karenzone @vishaangelova wanted to bring this to your attention. thank you

[elasticmachine]

Pinging @elastic/ingest-docs (Team:Docs)