📖 Add issue template for release tasks

[lentzi90]

What this PR does / why we need it:

This adds an issue template for tracking release activities, and updates the release docs with post-release tasks.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2569

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• if necessary:
  • includes documentation
  • adds unit tests

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