Experimental: support disabling pretty-urls (for readthedocs)

[ebolyen]

A potential solution to the assumption of URLs without extensions (pretty urls) being served from a bare html file noted in jupyter-book/mystmd#984.

Limitations:

• First time working with this theme, so I probably missed something.
• No tests yet (want to get feedback on the approach)
• I've only changed the "book" theme.
• Doesn't handle the flyout menu in readthedocs, I think that may only need a react upgrade, which I will probably test out soon.

Approach

• Add a site option for pretty_urls, it is by default interpreted to be true. (I'm not sure if there's a better way to cause a default, but it didn't seem like it since we're relying on the remix loader which just fetches the config.)
• When false, the book theme will set a React context which various components will look at.
• I looked at each reference to base_url and worked out if it was relevant. (There is one section that uses base_url in a way that I think might be a bug, but I'm not familiar enough with what "internal" means to this code.)

Alternative

• Could use MODE == 'static' instead of a new site option, but I think the URL extensions are technically orthogonal. Since some hosts will redirect, and others won't. You might even prefer .html for whatever reason.

Example
https://genome-sampler.readthedocs.io/en/latest/

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[ebolyen]

Thanks @changeset-bot, I'll switch this to a draft.