Make release cycle charts reusable

[hugovk]

Right now, the release cycle SVGs are monochrome when viewed as standalone images:

This is because all the colours are defined in the website's own .css file.

This PR moves the colours into the SVG files themselves, except for the theme-aware overrides using things like --color-foreground-primary from Furo, needed for dark/light mode support.

Also serve these images, so we can display them on https://www.python.org/downloads/ where our end users visit, and not just in the CPython developer's guide. Re: https://endoflife.date/recommendations#bad---python

Needed for PR: pythondotorg#2837

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📚 Documentation preview 📚: https://cpython-devguide--1708.org.readthedocs.build/

[AA-Turner]

Why is this needed? Semantically, I think we should prefer static path to extra path: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_extra_path

A list of paths that contain extra files not directly related to the documentation

A

[hugovk]

Why is this needed?

So that they're served at https://devguide.python.org/release-cycle.svg

Semantically, I think we should prefer static path to extra path: sphinx-doc.org/en/master/usage/configuration.html#confval-html_extra_path

A list of paths that contain extra files not directly related to the documentation

A

Okay, so have the generate script save them to _static, and have them served at https://devguide.python.org/_static/release-cycle.svg ?

I've pushed a commit for this.

[encukou]

Would it make sense to provide light, dark, and “responsive” variants?
More generally: is this just for python.org, or do we want others to easily reuse this?

If we decide that serving 3×2 static images, rather than 2, is worth it, I'd prefer doing the related changes myself -- I can clean up and adjust the CSS while doing that.

[hugovk]

Right now this is just for python.org which only has a light theme.

No objections to making more variants in the future, and I'm happy for you to do it.

What would a "responsive" variant be? The SVG is already, well, scalable :)

[encukou]

What would a "responsive" variant be? The SVG is already, well, scalable :)

Responding to the user's light/dark preference :)

I've sent my suggestions here: hugovk#36