Add Worker Versioning doc

[Sushisource]

What does this PR do?

Adds conceptual documenatation on the new Worker Versioning feature

Notes to reviewers

• Is there anything we can do to keep this hidden until it's released, or should we just not merge until then?
• There are TODOs about other sections to add, probably makes sense to add them to this PR, but I'll leave that in your hands

[rachfop]

[note to myself] This awaits version 1.19 before merging.

[djmagee]

I converted this PR to draft to ensure that it doesn't accidentally get merged too early, and I've started a terminology-and-style review.

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Name	Status	Preview	Updated
temporal-documentation	✅ Ready (Inspect)	Visit Preview	Sep 29, 2022 at 6:50PM (UTC)

[rachfop]

Created a PR off this branch to include it in the docs: https://github.com/temporalio/documentation/pull/2109 Awaiting docs approval first.