Make the nav sidebar's scroll-to-current-page behaviour less jumpy

[tsibley]

This avoids scrolling the sidebar when the link is already in view by
using "nearest" for the vertical axis ("block") instead of the default
of "start". This behaviour is much better when clicking on items in the
current viewport, especially if the current viewport matches the initial
viewport.

It's still a little jumpy when the current link isn't visible in the
initial viewport, because the sidebar position still changes across page
loads. But the new behaviour solves the big problem on initial home
page loads with a long sidebar where the logo and project title would be
immediately scrolled out of view.

Related to #824.

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Quick video demonstrating before/after.

[nienn]

Hi @tsibley, thank you for submission!
This probably needs more in-depth testing with multiple platforms and edge cases, but I think it can greatly improve the user experience with the sidebar.

[tsibley]

@nienn Yep, understood about testing! Data on caniuse.com shows pretty good support for {block: "nearest"} relative to browser-usage across the board, with the notable exception of Safari (on macOS and iOS).

I suspect (but haven't tested) that when the options object parameter isn't supported, as in Safari, it will be treated as a truthy value for the alignToTop boolean parameter and so will in effect act like the current behaviour.

Would be good to test all this empirically, of course. Does RTD have the resources to do this?

[tsibley]

Repushed after rebasing onto the current tip of master and also regenerating the built theme.js asset so it matches the source file.