[Security Solution] Create test plan for prebuilt rule installation a…

…nd upgrade workflows (elastic#160685)

## Summary

Combines the two following test plans into one:

- [Test plan for the legacy workflows of installing/upgrading prebuilt
rules](https://docs.google.com/document/d/1d_1DYnHlnCaPznWTjeCxhoaRUwxc2O_V0LToAPG0xLE/edit)
- this google doc is deprecated and will be replaced by the consolidated
test plan created in this PR
- [Test plan for the new workflows of installing/upgrading prebuilt
rules](https://docs.google.com/document/d/1cYvwtpzk0uLn5R88BlCX-fRIwR2n_NoqkSjIXkY9k34/edit)
- this google doc is deprecated and will be replaced by the consolidated
test plan created in this PR

Adds a template for writing new test plans.

---------

Co-authored-by: Georgii Gorbachev <[email protected]>
Co-authored-by: Kibana Machine <[email protected]>

x-pack/plugins/security_solution/cypress/test_plans/README.md

@@ -2,7 +2,7 @@
 
 This folder contains test plans for the features of Security Solution.
 
-## Folder Structure
+## Folder structure
 
 The folder is first split into major Security Solution domains:
 
@@ -28,6 +28,15 @@ Within each subdomain, you can organize test plans as you like, for example:
 - you might want to have a folder per feature, if your features are large and you have multiple test plans per feature
 - or you might want to have a plain list of test plans if features are relatively small
 
-## Ownership
+## Folder ownership
 
 Each subdomain folder should be owned by a single GitHub team in the `.github/CODEOWNERS` file.
+
+## Test plan structure
+
+Some examples for reference:
+
+- [Test plan template](./test_plan_template.md).
+- [Installation and Upgrade of Prebuilt Rules](./detection_response/prebuilt_rules/installation_and_upgrade.md).
+
+Feel free to tune the structure whenever it makes sense and improves readability or maintainability of your plan: add more sections to `Useful info`, add more top-level sections in addition to `Useful info` and `Scenarios`, etc.