Guide for Embed API

[humitos]

Document two different use cases for the Embed API.

My goals with this PR are:

• communicate that Embed API exists
• show two different ways of usage with real examples
• let the user know that they can build something new on top of it
• promote sphinx-hoverxref
• have clickable working examples

I'm not mentioned https://github.com/agjohnson/readthedocs-client-js because I think it needs more work or at least a review and testing that it's still working. I think it makes sense to just mention and AJAX call for now and then we can update this page when we have polished our readthedocs-client-js.

Rendered version https://docs--7089.org.readthedocs.build/en/7089/guides/embedding-content.html

[ericholscher]

How does this compare to #7000?

[humitos]

Haha, I totally forgot about that PR 😄. However, I'd avoid mentioning our JS client for now (as #7000 does) since we haven't touched or polished yet. Besides, I think this guide is more complete, IMHO.

Please, take a look at this and let me know which you prefer.

[ericholscher]

I've pushed some updates here. I moved the hoverxref section up since it's the most functional for users. I found the screenshot I found really confusing. I'd crop it more. I reduced the width, since I got confused when reading the page and thought it was the actual page content. The hover also looks like it's coming from the code sample, which was really confusing.

[humitos]

I changed the image using one from our own documentation and centered it. I think it looks more differentiated from the page's content itself and better because the tooltip is nicer :)

[ericholscher]

We should also add documentation for the embed API to the v2 public API docs, but this is fine for a first step I think.