Add additional dev guides

[bmorelli25]

Summary

For elastic/integrations#9796.

Adds a few additional developer guides to the IDG. Not changing the content in this PR. Just copying over 1:1 and adding to the TOC where it makes sense.

• developer_workflow_bug_fix_older_package_version.md
• developer_workflow_fleet_ui.md
• fine_tune_integration.md
• import_from_beats.md
• tips_for_building_integrations.md

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[mergify]

This pull request does not have a backport label. Could you fix it @bmorelli25? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

LGTM! 🦒
Great idea to just move in the content verbatim, so we can proceed with removing the .md content. So nice.