Update theme (shortcuts improvements)

[jarrodmillman]

The shortcuts/sidebar has been fixed. It now doesn't scroll up with the rest of the page and the links work. It also makes the appearance more like what you see with pydata-sphinx-theme.

Old look

New look

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[stefanv]

[stefanv]

I should note that only h headers with ids will appear on the sidebar, and I see some of the NumPy headings don't; so that may be something else to fix in this PR, @jarrodmillman.

[jarrodmillman]

I would rather keep the style and content changes separate. Maybe someone could fix the headings in a separate PR.

[stefanv]

That works!

[stefanv]

The sidebar generation is now much more consistent than it used to be, and I fixed some scrolling artifacts along the way. Let's merge this and if any issues arise we can fix those later. "Headings" (<p><strong>...</strong></p>) that don't show up in the sidebar will have to be changed to proper headings with IDs (<h2 id="something">something</h2>). This is the way Hugo generates headings by default, so as long as text is written in Markdown it will work out of the box. Hand-crafted HTML will need updating, but that was the case before this PR already.

[jarrodmillman]

Just to be clear, the headings issue @stefanv mentioned is unrelated to this PR. The headings haven't changed.