Add config option to make version warning banner sticky

[melissawm]

Hello, friends!

We at napari just noticed that if we link to a section header on a page on an old version of the documentation, the version warning banner is not visible, since it does not move with the page scroll. For example, navigating to

https://napari.org/0.6.0/api/napari.html#napari.save_layers

there is no indication (apart from the version switcher) to note that this is an old version.

Is it possible to have a config value that turns the version warning into a sticky banner that will show up even in section header anchor URLs?

I am happy to submit a PR for it if you think it's a good idea.

Thanks!

[Carreau]

If you can submit a PR, please do; we've been trying to go through the backlog and pushing for a 0.17.0 (long overdue), we may push 0.17.0 out soon and make a 0.18 in a few weeks/month instead of constantly chasing the new improvement issues.