Add warning when browsing developer docs

[stefanv]

This adds a warning banner to the top of the current "article" when the dev docs are being browsed (perhaps you want to look at stable docs?).

Unfortunately, I cannot figure out how to coax pydata-sphinx-theme into rendering the breadcrums either underneath or above the block.

Any ideas?

/cc @lagru

[lagru]

The theme has the option to display an announcement banner which is displayed at the top of the screen, but will disappear once they start scrolling. I think that would be a perfect fit for what you want to achieve?

[lagru]

Additionally, we could also color the version switcher like SciPy does. That might be another hint to the user.

[lagru]

By the way, how is this relevant to the 0.21 release? It shouldn't be affected by this change in any way.

[lagru]

Should I update the PR with my suggested changes?

[stefanv]

Can you make the banner only appear on the dev version? Ah, yes, that should be easy since it happens in conf.py. Go ahead and give it a try.

[jarrodmillman]

We use these templates in networkx:

• https://github.com/networkx/networkx/blob/main/doc/_templates/layout.html
• https://github.com/networkx/networkx/blob/main/doc/_templates/dev_banner.html
• https://github.com/networkx/networkx/blob/main/doc/_templates/eol_banner.html

When I make a release, I do the following in the same commit where I update the release version

git rm doc/_templates/layout.html

Then I add it back when I bump the release version after the release.

I haven't been good about it recently, I should update the NX release process documentation so I don't forget. But then I am supposed to manually insert

<div class="admonition warning">
<p class="admonition-title">Warning</p>
  <p>This documents an unmaintained version of NetworkX. Please upgrade to a maintained version and see <a href="https://networkx.org/documentation/stable/">the current NetworkX documentation</a>.</p>
</div>

In the past, I just use find and awk to insert the text from the command-line. I will just add the one-liner to the release process documentation or maybe to the README in the documentation repo.

At any rate, it would make sense to make this consistent between projects.

[stefanv]

This is now supported by pydata-sphinx-theme; we just need to activate that feature.

[lagru]

Continued in #7139.