Pin the logo, project name, version, and search box to the top of the sidebar

[tsibley]

…when it scrolls. This helps maintain the page's visual structure even if the sidebar navigation is long and requires scrolling.

Testing

• [x] Built docs.nextstrain.org locally with this theme change and clicked around.
• [x] Clicked around the demo docs, e.g. https://nextstrain--28.org.readthedocs.build/projects/sphinx-theme/en/28/demo/long.html#example-menu-9

[tsibley]

Two downsides I noted which I think are ok even if not ideal:

• Without a visual separation/layering of the top of the sidebar from the navigation menu, it's not always clear that the menu sidebar scrolled. This is kinda the case already, even without this change though, and scrolling the page will still scroll the menu (they're "locked" together by default).

• A persistent scrollbar alongside the nav menu could help provide the visual indication, but it seemed out of place and also currently misleading about the effective scrollable area (attached to the whole sidebar because that's the scroll element, but only the nav menu actually scrolls).