[DOCS] Move reference pages out of appendix for migration

[lcawl]

This is one of a sequence of PRs necessary to move the machine learning function reference pages out of an appendix in preparation for migration. The first step is to change the way the source files are referenced in the Machine learning guide.
The next PR will make the appropriate changes (removal of the appendix and exclude clauses) from the source files in the elasticsearch repo.
Then then final PR will remove the redundant copies from the stack-docs repo.

This PR also moves the supplied configurations out of the appendix as well so they can be migrated.

[github-actions]

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[wajihaparvez]

LGTM! 🚗