[8.15] Document ways to provide multiple Agent uninstall tokens (backport #6300)

[mergify]

Resolves #5865.

Previews

ESS:

• Uninstall Elastic Agent | Provide multiple uninstall tokens – new section
• Prevent Elastic Agent uninstallation | Access uninstall tokens – tip added at the end of the section

Serverless:

• Uninstall Elastic Agent | Provide multiple uninstall tokens – new section
• Prevent Elastic Agent uninstallation | Access uninstall tokens – tip added at the end of the section

  ---

  This is an automatic backport of pull request Document ways to provide multiple Agent uninstall tokens #6300 done by Mergify.

[mergify]

Cherry-pick of 87e3acb has failed:

On branch mergify/bp/8.15/pr-6300
Your branch is up to date with 'origin/8.15'.

You are currently cherry-picking commit 87e3acb9.
  (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/getting-started/agent-tamper-protection.asciidoc

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   docs/getting-started/uninstall-agent.asciidoc
	deleted by us:   docs/serverless/edr-install-config/agent-tamper-protection.asciidoc
	deleted by us:   docs/serverless/edr-install-config/uninstall-agent.asciidoc

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

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