…part 1 (elastic#211300)

**Epic:** elastic#174168
**Partially addresses:**
elastic#202068,
elastic#202078,
elastic#202079

## Summary

We're cleaning up and refactoring our existing test plans for prebuilt
rule customization, upgrade, and export/import workflows.

Specifically, in this PR we're consistently:

- Changing the file names and test plan titles.
- Adding or expanding test plan summaries.
- Regenerating tables of contents using the [Markdown All in
One](https://marketplace.visualstudio.com/items?itemName=yzhang.markdown-all-in-one)
VS Code extension.

No "functional" changes have been made to any test plans, such as
adding, removing, or updating any scenarios.

This refactoring prepares the test plans for being "functionally"
changed and improved in follow-up PRs. For example, we're going to cover
the logic described in elastic#210358
and address any other gaps in coverage.

(cherry picked from commit 9600de4)