[YAML] Add documentation to Beam site.

[robertwb]

This moves the content from the yaml subdirectory of the Python SDK to the Beam site itself.

There are several references out there to the original files. I'll remove and add redirects in a future PR once these changes are live.

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[robertwb]

R: @Polber

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[Polber]

R: @VeronicaWasson

[Polber]

LGTM, but want to give Veronica a chance to look