…part 2 (elastic#211472)

**Epic:** elastic#174168
**Partially addresses:**
elastic#202068,
elastic#202078,
elastic#202079
**Follow-up to:** elastic#211300

## Summary

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

Specifically, this PR:

- Creates an "entrypoint" file that should help navigate all the test
plans for prebuilt rules.
- Creates a file for keeping common information about prebuilt rules
that can be shared between the test plans.
- Extracts duplicated terminology to the file with common information.
- Extracts duplicated assumptions to the file with common information.
- Extracts duplicated non-functional requirements to the file with
common information.
- Adds user stories to each test plan.
- Updates links to tickets in every test plan.

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.

## Review tip

It might be easier to review this PR commit-by-commit as each of them
contains logically cohesive changes.

(cherry picked from commit 786df79)