[8.15] Edit required_fields field for custom rules in UI [classic] (backport #5287)

[mergify]

Contributes to #5131.

Previews

• Classic: Create a detection rule - There's a new step in every rule type procedure except for machine learning, which doesn't support the "required fields" feature.

• Serverless: Click the link below, then navigate to the page above in the Serverless Security doc set.

Related

• I'm expecting a conflict between this PR and Edit related_integrations field for custom rules in UI [classic] #5151, since both PRs change the same lines in the same file. Will need to resolve by including both steps in the correct order that they appear in the UI (first Required fields, then Related integrations).
• https://github.com/elastic/staging-serverless-security-docs/pull/357 was the original serverless twin for these changes, but I made additional edits to serverless content in this PR.

---

This is an automatic backport of pull request #5287 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of ff8d574 has failed:

On branch mergify/bp/8.15/pr-5287
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit ff8d5749.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/detections/rules-ui-create.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/rules/rules-ui-create.mdx

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

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