conf.py: Set navigation_with_keys to allow navigating documentation through arrow keys

[C0rn3j]

Feature from sphinx-doc/sphinx#2064.

[joshtrichards]

Nice idea. Seems to work reasonably.

The only concern I've come up with: it disables the ability to use the keyboard to scroll sideways when there are big tables (e.g. https://docs.nextcloud.com/server/latest/admin_manual/ai/overview.html). The horizontal scrollbar still works there of course.

I don't consider that a showstopper, as I consider those wide tables already to be in need of some other clarity to indicate they can be scrolled (since the scrollbar doesn't even appear - to indicate they're scrollable - unless you roll your cursor in front of the table).

So I'm okay with this. Not sure how others feel about it so I'll leave this PR open a bit before merging so that others can weigh in first.

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