Delete mdx and docnav.json files

[colleenmcginnis]

Related to #6061

Removes all .mdx and .docnav.json files now that serverless docs are built with AsciiDoc (https://www.elastic.co/guide/en/serverless/current/what-is-security-serverless.html).

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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.