[mdx → asciidoc] Add AsciiDoc serverless files

[colleenmcginnis]

Related to elastic/docs#3096

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-security-serverless.html

[github-actions]

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[mergify]

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

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticdocs]

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[joepeeples]

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