Improvements to LH sidebar layout

[dstansby]

This improves the layout of the donate button by stretching it to fill the full width of the sidebar. In order to test this I've also added the current donate button sidebar from https://github.com/matplotlib/matplotlib. This adds a dependency of sphinx-design here. I've done this in the spirit of keeping all the code for the donate button in mpl-sphinx-theme, so it can be used by other projects, not just the main Matplotlib docs.

In conjunction with matplotlib/matplotlib#29789, this improves the layout:

Before:

After:

[timhoffm]

Can we keep a larger margin between the cheetsheet and the support button? They are not related and thus should be cleary separated.

[dstansby]

Can we keep a larger margin between the cheetsheet and the support button? They are not related and thus should be cleary separated.

👍 - I realised that the cheatsheet sidebar is specific to the main Matplotlib docs, so I made this change (and other cheatsheet changes) at matplotlib/matplotlib#29789

[jklymak]

It would be preferable if the left hand gutter didn't grow relative to the main content

[dstansby]

It would be preferable if the left hand gutter didn't grow relative to the main content

Do you mean the LH sidebar changing width when the browser window is resized? Looking at the CSS I think that's baked into pydata-sphinx-theme, so would be a request to change upstream, and I think out of scope of this PR anyway (which deals with how our content fits inside the sidebar, not the sidebar itself).

[jklymak]

Your two screen shots make it seem the LHS sidebar has gotten wider. Perhaps you just changed the width of your browser window?

[dstansby]

Ah that's just because the 'after' screenshot is cropped off a bit on the right (you can see the twitter icon is cropped out). In browser/real life they're the same width.