Enable NAVIGATION_WITH_KEYS by default

[abitrolly]

navigation_with_keys is a hardly discovered feature of Sphinx JS. Even official Python docs theme doesn't have it enabled with "The kitchen sink is a showcase of every Sphinx feature:" - https://sphinx-themes.org/sample-sites/python-docs-theme/kitchen-sink/

There were some arguments in the past that shortcuts break people expectations. I won't link to these arguments, because they were not backed by UX studies or user voting, FWIW), but if you have some prooflinks that having no navigation keys by default are better for users, please feel welcome to add it here. In the meanwhile looks how responsive and beautiful Rust documentation experience is:

1. Open https://doc.rust-lang.org/book/
2. Press Left, then PageDown
3. Repeat until happy

Please let us have some discussion, so that we can make it opt-out for themes where it breaks things, rather than off limits for everybody.

https://www.sphinx-doc.org/en/master/usage/theming.html#:~:text=navigation%5Fwith%5Fkeys

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Some back story - I discovered the NAVIGATION_WITH_KEYS option when I was trying to add shortcuts to the RTD theme https://sphinx-rtd-theme.readthedocs.io/en/stable/configuring.html - it is my at least 2nd serious attempt to do this, but in my previous life I meddled a lot with Sphinx internals, so the unconscious thought of doing this was probably hitting me few times a week over the past decade. Today I Learned that RTD has all the JavaScript enabled for this feature, but no docs for users to know how to turn it on.