[8.x] Custom roles & privileges: Revise, reorder, add serverless requirements pages (backport #5779)

[mergify]

Contributes to https://github.com/elastic/security-docs-internal/issues/11.

Also incorporates changes originally drafted in #5763 and #5772, but consolidated into this branch.

Previews

Preview serverless

Generating a preview is tricky, because a lot of content currently is hidden behind the serverlessCustomRoles docs feature flag until the feature is available in Production (ETA TBA). I temporarily commented out the conditionals to show the content for review purposes, and this is the latest preview build without the conditionals:

https://elastic-dot-co-docs-production-bc2c75hpv-elastic-dev.vercel.app/current/serverless/security/what-is-security-serverless

I then un-commented the conditionals to re-hide the new content behind the flag. (This is more complicated than preferred, but I'd rather get this content merged soon — even with the added complexity — so we can continue to build around it, versus the content only existing in a PR branch and getting stale).

• To preview the content that will eventually get published once the custom roles feature is available in Prod, use the link above and navigate to the respective serverless pages:
  • Elastic Security requirements
  • Elastic Defend feature privileges
  • Cases requirements
  • Entity risk scoring prerequisites
  • Network map data requirements
  • Machine learning job and rule requirements - Very minor edits; actual requirements still very unclear but can be documented further in a separate issue/PR.
  • Advanced settings
  • Endpoint response actions

Preview ESS

These pages have minor edits for consistency with serverless counterparts:

• Elastic Security requirements
• Detections requirements
• Elastic Defend feature privileges
• Cases requirements
• Entity risk scoring requirements
• Machine learning job and rule requirements
• Configure network map data
• Advanced settings

Summary of changes

Adding content

For serverless MVP, we omitted a lot of content because custom roles and feature privileges weren't available yet. We can add that content back in now. As much as possible, I've tried to make the serverless content match ESS exactly, but there are spots where we also need to mention serverless-specific predefined roles (since serverless customers can use either predefined or custom roles now).

In some areas, I also tried not to introduce granular differences like changing Kibana to Elastic Security, because our assumptions have changed about that (the term still appears all over the serverless UI). Unifying content across serverless and ESS is a priority now.

Reordering pages

We no longer need a separate section of requirements pages in serverless, so this PR moves those pages to locations that match ESS docs: each page is within a feature section.

Aligning titles

Our requirements pages currently go by lots of names: some are "prerequisites," some are "requirements," and one is even "prerequisites and requirements"! This PR uses a single unified name for all pages (for example, "Detections requirements"), in both ESS and serverless.

Housekeeping

This PR also moves around the serverless source files so they'll be easier to find in their respective feature folders. Doesn't affect output but improves authoring experience.

---

This is an automatic backport of pull request #5779 done by Mergify.

[mergify]

Cherry-pick of 9b556a7 has failed:

On branch mergify/bp/8.x/pr-5779
Your branch is up to date with 'origin/8.x'.

You are currently cherry-picking commit 9b556a7b.
  (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/advanced-entity-analytics/ers-req.asciidoc
	modified:   docs/cases/cases-req.asciidoc
	deleted:    docs/cases/images/case-feature-privs.png
	modified:   docs/detections/detections-req.asciidoc
	modified:   docs/getting-started/advanced-setting.asciidoc
	modified:   docs/getting-started/defend-feature-privs.asciidoc
	deleted:    docs/getting-started/images/case-feature-privs-example.png
	deleted:    docs/getting-started/images/case-feature-privs.png
	deleted:    docs/getting-started/images/endpoint-privileges.png
	modified:   docs/getting-started/net-map-req.asciidoc
	modified:   docs/getting-started/sec-app-requirements.asciidoc
	new file:   docs/serverless/advanced-entity-analytics/ers-req.mdx
	new file:   docs/serverless/edr-install-config/defend-feature-privs.mdx
	new file:   docs/serverless/investigate/case-permissions.mdx
	new file:   docs/serverless/rules/detections-permissions-section.mdx
	new file:   docs/serverless/sec-requirements.mdx

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/serverless/advanced-entity-analytics/ml-requirements.mdx
	deleted by us:   docs/serverless/endpoint-response-actions/response-actions.mdx
	deleted by us:   docs/serverless/explore/conf-map-ui.mdx
	deleted by us:   docs/serverless/serverless-security.docnav.json
	deleted by us:   docs/serverless/settings/advanced-settings.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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