[mdx → asciidoc] Add AsciiDoc serverless files

[colleenmcginnis]

Description

Adds AsciiDoc serverless files.

I did a full review where I looked at:

• The diff between the MDX and AsciiDoc file for each page and confirmed that the only changes were to the syntax.
• The rendered result of the both the MDX version and AsciiDoc version of each page side-by-side and confirmed that they result in the same structure.

This PR adds AsciiDoc files and images to replace DocIcon. A few MDX files are edited where the MDX syntax was incorrect. MDX files will be deleted in a follow-up PR.

You can see the result in this preview: https://docs_bk_3096.docs-preview.app.elstc.co/guide/en/serverless/current/what-is-observability-serverless.html

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Related to elastic/docs#3096

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

[github-actions]

A documentation preview will be available soon.

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[elasticdocs]

🚀 Built elastic-dot-co-docs-preview-docs successfully!

• https://elastic-dot-co-docs-production-4yor8dpvq-elastic-dev.vercel.app

• Built with commit 0cb766c

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