Add a small callout alerting users to the new site

[ericholscher]

This is just a mockup, happy to have additional design advice. I think this could be a "banner" style or similar, and perhaps only show when we have the ref=dotcom header if there's an easy way to do that.

Preview

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:books: Documentation preview :books:: https://read-the-docs-website--181.com.readthedocs.build/

[ericholscher]

@agjohnson I'd like to get something like this put together for when we migrate .org users to the new website. I know Benjamin was worried about folks being worried about Community going away, so we should try to clarify this as much as possible.

[agjohnson]

I pushed up an option at 1be48d7

It does move the call out to only on the homepage, as putting it on all pages could alter those pages, and the block isn't dismissable.

The copy could be improved there, but I stuck to using a single line (to avoid altering the page layout above the fold too much. I updated the link as well, but we will want to update that blog post around the same time too.

[ericholscher]

This looks like a nice approach! I think this looks good for when we redirect .org, with the content updates you mentioned 💯

[agjohnson]

I'll set this to blocking until we get the blog post updated and the link updated here if needed.

[ericholscher]

This should now be unblocked with the updates here: https://github.com/readthedocs/blog/pull/236