Make it easier to preview Registry documentation changes from PRs

[melinath]

Currently it's a multi-step process to preview documentation, which either requires generating the providers (which is our current recommendation, even for docs-only changes) or navigating GitHub to get from the PR to the auto-pr branch diff to the modified markdown files, and knowing you can open them on the branch.

Once you have the docs content, you need to then copy-paste it into Hashicorp's doc preview tool: https://registry.terraform.io/tools/doc-preview

It would be great to simplify this, for example:

1. by providing easy links directly to preview any generated docs in the GH interface. Although it's not the same as the Hashicorp preview, it would still be an improvement over the current state, and would reduce the effort to put the docs on Hashicorp's site.
2. By providing a link that pre-fills the preview box on the preview site (probably not possible)
3. By providing a link to some other page that renders a preview
4. By somehow creating a screenshot of the preview for the page(s) and sharing a link to the screenshot

Any implementation for this should take into account that we ideally don't want to increase the length of the diff comment.

b/322343045