[DOCS] 8.2.1 Release Notes

[nastasha-solomon]

Description

Generate the 8.2.1 release notes and publish them before or on the release day, which is May 17, 2022.

Set up the release notes file

Follow these steps

1. Check out the main branch.
2. Run a git fetch origin then git pull to pull in the latest changes.
3. Create a branch for the release notes, for example, issue-XXX-8.2.1-rn.
4. Add a link to the 8.2.1 release notes to update the release note's index page. Should be:
   <<release-notes-8.2.1, {elastic-sec} version 8.2.1>>
5. Open a PR.
   NOTE: Remember to add the 8.2 label so the release notes are backported to the proper branches.
6. Complete the technical and editorial review.
7. Merge the PR to main and merge the backports after Mergify creates them.

[nastasha-solomon]

elastic/kibana#131363 needs to be documented in the RN.