[WIP] open preview docs

[sebastian-lerner]

trafficstars

it's a lot. it's all from here: https://circleci.atlassian.net/wiki/spaces/Runner/pages/6551503582/Closed+Preview+Customer-Facing+Instructions

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[sebastian-lerner]

@devinmarieb when we push this live, can we also add a note to https://circleci.com/docs/runner-on-kubernetes that points to this new page that says something like:

"this new container agent replaces this kubernetes runner, it's now in open preview, check it out..."