DOC: Restore banner indicating docs are unreleased

[QuLogic]

PR summary

This was dropped when switching to the pydata-sphinx-theme. They have a method of adding an announcement, but because it's a pure text substitution, we can't have it automatically search like our old banner.

While the theme also supports loading from an http resource, that would mean writing some JavaScript instead of automatically creating this div at build time.

So override the theme component, and create the search link at build time.

I'm only a bit annoyed that the announcement overlays the normal navigation bar. I'm not sure it's really possible to avoid that without wrapping both in their own sticky div, but that would require overriding the entire theme layout, and that's a bit too much for my taste.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• [n/a] new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[QuLogic]

I'm only a bit annoyed that the announcement overlays the normal navigation bar. I'm not sure it's really possible to avoid that without wrapping both in their own sticky div, but that would require overriding the entire theme layout, and that's a bit too much for my taste.

For this bit, I've added in a shift to the normal navigation bar based on its minimum height from CSS. This won't work everywhere (i.e., if text is wrapped), but is a bit better than nothing.

[melissawm]

There is an un-merged implementation of this in the pydata-sphinx-theme: pydata/pydata-sphinx-theme#1335

I have actually copy-pasted a previous solution also from the pydata-sphinx-theme here: napari/napari-sphinx-theme#132

Ultimately the napari team is going in another direction but I just wanted to point out existing implementations in case this helps.

[QuLogic]

I was aware of the pydata-sphinx-theme PR; I'm hoping we can migrate to it once it's been released. I think we won't have a reason to put an announcement on the devdocs, so I think it's okay to steal that space to make the warning.

Anyway, we already have people confused about unreleased features, so it's probably best to restore this sooner rather than later.

[ksunden]

Do we have access to all variables from conf.py here? if so would it be better to do is_release_build here?

matplotlib/doc/conf.py

Line 34 in 1cc50f6

is_release_build = tags.has('release') # noqa

[QuLogic]

@tacaswell also noted that we may want to do announcements on releases, and this doesn't quite allow that, but I think I can make a small tweak to fix that.

[QuLogic]

OK, should work in all 3 cases: release build, unreleased build, and an announcement.