Docs: generate social media cards per page

[hugovk]

For python/docsbuild-scripts#147, followup to #129085, partner to python/docsbuild-scripts#242.

Let's check the performance of this on 3.14 first, to make sure the extra images don't slow the build up too much or take up too much space.

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📚 Documentation preview 📚: https://cpython-previews--129120.org.readthedocs.build/

[hugovk]

Here's the docs for the social cards: https://sphinxext-opengraph.readthedocs.io/en/latest/socialcards.html

This generates a Doc/build/html/_images/social_previews/ directory of 509 PNG images taking 40.9 MB, like summary_library_mm_8b5ad9c4.png:

It removes HTML from the page head like:

<meta property="og:image" content="https://docs.python.org/3/_static/og-image.png" />
<meta property="og:image:alt" content="Python documentation" />
...
<meta property="og:image:width" content="200" />
<meta property="og:image:height" content="200" />

And adds:

<meta property="og:image:width" content="1146" />
<meta property="og:image:height" content="600" />
<meta property="og:image" content="https://docs.python.org/3/_images/social_previews/summary_library_mm_8b5ad9c4.png" />
<meta property="og:image:alt" content="The modules described in this chapter implement various algorithms or interfaces that are mainly useful for multimedia applications. They are available at th..." />
...
<meta name="twitter:card" content="summary_large_image" />

There's a problem: we're referring to a file at https://docs.python.org/3/_images/social_previews/summary_library_mm_8b5ad9c4.png when it will really be at https://docs.python.org/3.14/_images/social_previews/summary_library_mm_8b5ad9c4.png for 3.14.

This comes from ogp_site_url = 'https://docs.python.org/3/'

It didn't matter before that all versions referred to the static https://docs.python.org/3/_static/og-image.png logo.

[hugovk]

Build times:

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Images are only built when the create-social-cards tag is set, currently only on the docs server: python/docsbuild-scripts#242. We'll check build times on the server after this is merged.

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The default RTD build time doesn't change because the create-social-cards tag isn't set so it doesn't create images. But for indicative times:

• Without images: 2m 22s
• With images: 3m 0s

It might be useful to have them on RTD for testing social cards, but we can decide on that later.

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Locally we also don't build images for the same reason, but on macOS:

• Without images: 34s
• With images: 45s

I expect it would be slower on older systems, and there's not much need for them locally. It's not too hard to enable it if needed (don't define ogp_image in conf.py, add matplotlib to requirements.txt) and we can decide later if we want it locally.