[8.0](backport #30355) Add Beats upgrade docs for 8.0

[mergify]

This is an automatic backport of pull request #30355 done by Mergify.
Cherry-pick of c26cea6 has failed:

On branch mergify/bp/8.0/pr-30355
Your branch is up to date with 'origin/8.0'.

You are currently cherry-picking commit c26cea662a.
  (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:
	new file:   libbeat/docs/images/confirm-index-template.png
	new file:   libbeat/docs/tab-widgets/load-index-template-widget.asciidoc
	new file:   libbeat/docs/tab-widgets/load-index-template.asciidoc
	deleted:    libbeat/docs/upgrade-setup-commands.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/docs/upgrading.asciidoc

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

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[botelastic]

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[elasticmachine]

💚 Build Succeeded

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• Start Time: 2022-03-01T19:18:18.821+0000

• Duration: 113 min 6 sec

Test stats 🧪

Test	Results
Failed	0
Passed	37917
Skipped	3326
Total	41243

💚 Flaky test report

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🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.0/pr-30355 upstream/mergify/bp/8.0/pr-30355
git merge upstream/8.0
git push upstream mergify/bp/8.0/pr-30355

[dedemorton]

@elasticmachine run elasticsearch-ci/docs

[dedemorton]

/test