Post 2.50.0 Streamline release guide improvements

[lostluck]

This is a larger cleanup of the release guide to clarify timings and positioning of the broader steps.

Larger emphasis has been put on the Release Manager's role prior to the release cut. If issues are addressed in the main branch before the cut, they do not need to be addressed afterwards. The release manager is empowered to do so.

Clarify some timing around the begining and end of the release by calling out the preparation steps should be performed within the week prior to the release cut. Also call out that release finalization tasks can be done within a few hours of the release being completed.

While the guide isn't materially shorter, maintaining the simpler structure as in the overview will make it less overwheling to release managers, since it's clearer when the various subtasks apply, vs the previous "13" steps. This should clarify the "build release candidate + vote + fix" loop that is the bulk of a release manager's tasks. I believe this will help get to release candidates sooner.

Add a new section "Post Release Tasks" which is where the Playground update task lives. While it's part of the release tasks, it doesn't block the release overall, so it felt weird structurally to have it as important as "Build a Release Candidate".

Various minor typos and fixes.

Rendered version should become available here:

http://apache-beam-website-pull-requests.storage.googleapis.com/28287/contribute/release-guide/index.html

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[lostluck]

R: @damccorm @kennknowles @jrmccluskey @Abacn

[github-actions]

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

[damccorm]

Thanks for doing this!

[lostluck]

PTAL

[damccorm]

LGTM

[kennknowles]

I hate hate HATE markdown that is only readable when rendered (aka using a sequence of 1. 1. 1. 1.). But you didn't start the fire, for the most part...

[lostluck]

I hate hate HATE markdown that is only readable when rendered (aka using a sequence of 1. 1. 1. 1.). But you didn't start the fire, for the most part...

I'm right there with you, but I'd make that a separate change across the whole file rather than adjust the untouched lists here, or only partially convert it.