removing readthedocs.io references

[balopat]

We should not refer to readthedocs.io from our docs for content, unless it's absolutely necessary.
This removes the last readthedocs.io references.

[balopat]

Note that from md files we shouldn't use file extensions, but from notebooks we should - this inconsistency is weird, but that's how sphinx, recommonmark and nbsphinx work - I hope that this works with devsite as well, @lamberta?

[lamberta]

we shouldn't use file extensions, but from notebooks we should

Since we try to make the OSS GitHub browsing experience as pleasant as possible, include file extensions for everything (.md and .ipynb). Should work in Colab, as well. These extensions are stripped when imported into the site.

The exception is the _book.yaml nav file. No extension there since it's for the site only.

[dstrain115]

Do we need an extension for this (ipynb)? Same with below. Your comment suggests we should add it.

[lamberta]

Yes. You can test this in the github preview. This link doesn't work in GitHub unless you add variational_algorithm.ipynb
It will work on the site since we strip extensions anyway, but, as a general style, using the extension means the link works everywhere.

[balopat]

Okay, thanks for the input - I agree, I'll change this back to .ipynb.
However, this will need a bit more work due to a bug in recommonmark - I'm running into readthedocs/recommonmark#130.

The current version of this page https://cirq.readthedocs.io/en/latest/docs/tutorials/examples.html is broken both on RTD and in devsite. I'm having promising results with myst...