…part 3 (elastic#212420)

**Epic:** elastic#174168
**Partially addresses:**
elastic#202068,
elastic#202078,
elastic#202079
**Follow-up to:** elastic#211472

## Summary

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

Specifically, this PR:

- Updates the common structure of test plans:
- Adds a tip to each test plan's header to help with navigating all our
test plans for prebuilt rules.
- Introduces a new `Requirements` section to each one and consolidates
assumptions, technical requirements and product requirements under it.
- Each `Product requirements` section now contains relevant user stories
and other product details. In the document flow this is kept close to
the `Scenarios` section.
  - Updates the test plan template according to the updated structure.
- Moves the documentation about customizable and non-customizable rule
fields to the common info document.
- Restores the lost changes from commit
elastic@bde4612.

No "functional" changes have been made to any test plans, such as
adding, removing, or updating any scenarios. The only functional-like
changes were made to the user stories when creating/updating the product
requirements sections.

This PR wraps up the refactoring efforts aimed to prepare the test plans
for being "functionally" changed and improved in follow-up PRs. In the
following PRs, we're going to cover the logic described in
elastic#210358, address any gaps in the
coverage, and improve the scenarios structure and wording.

## 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 c961659)