[8.x] [DE Team][8.16] Case system action being added for rules (backport #5703)

[mergify]

NOTE TO SELF: DO NOT MERGE UNTIL 8.16 FF

Contributes to #5457

This PR contains the following changes:

• Within the ESS and Serverless docs, I removed screenshots of the rule action connectors and response actions. These screenshots don't add significant value to the instructions for selecting an action and need to be frequently maintained.
• Made the following changes to the Create a detection rule page:
  • Revised step 5 at the start of the page so it reflects that users can set up actions for the rule
  • Changed the title of the alert notifications section from "Set up alert notifications (optional)" to "Set up rule actions (optional)"
  • Added a blurb about the case connector and linked to the connector docs
  • Moved the response actions section one more level down so it's under the "Set up rule actions (optional)" section. This seemed appropriate, seeing that response actions are considered a sub-type of rule actions.

Corresponding Serverless PR: #5615

---

This is an automatic backport of pull request #5703 done by Mergify.

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[nastasha-solomon]

run docs-build

[lcawl]

run docs-build