[docs] Migrate docs from AsciiDoc to Markdown

[colleenmcginnis]

Migrate docs from AsciiDoc to Markdown. The preview can be built after #6553 is merged.

Notes:

• More files are deleted than added here because the content from many files was moved to elastic/docs-content.
• When you get the build working and click the View deployment button, you'll need to add /reference to the end of the URL you're sent to in order to see a rendered page.

[github-actions]

A documentation preview will be available soon.

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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.