Update docs about preview from pull/merge requests

[stsewd]

I can create a redirect after this is approved. I simplified the content from the guide.

[ericholscher]

This seems useful, but we need to clean up the wording here to make it less clunky.

We have links to both .org and GitHub, whereas we support this feature on both platforms, and on both VCS providers. We need to figure out a way to make this cleaner.

[ericholscher]

This should be above the Warning banner, it's a much bigger feature. I think we can probably just remove the warning banner section, since it's not really that useful to document?

[stsewd]

I think we should document it somewhere, as people some times expect this kind of fetatures to be on mkdocs projects as well. #7810

[ericholscher]

Is this true? I thought search just didn't work :/

[stsewd]

It should just work, as they are regular versions, we can enable it by removing this line

readthedocs.org/readthedocs/projects/tasks.py

Lines 1318 to 1320 in 74df71d

	# Don't index external version builds for now
	if not version or version.type == EXTERNAL:
	return

[stsewd]

now with the custom options for search I think this is useful, so users can test the results.

[ericholscher]

Yea, it will just destroy our search indexes. I don't want to break prod search by indexing PR's, so we probably want a different cluster for it, if nothing else.

[ericholscher]

This looks great 👍

[stsewd]

redirect is set https://docs.readthedocs.io/en/latest/guides/autobuild-docs-for-pull-requests.html