Generate social media preview cards for the documentation

[AA-Turner]

Replaces #129120, requires python/docsbuild-scripts#242, closes python/docsbuild-scripts#147.

This generates a Doc/build/html/_images/social_previews/ directory of ~500 .png images (one per page; ~40MB). Per Hugo's comment in GH-129120, 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:url" content="https://docs.python.org/3/library/mm.html" />
...
<meta property="og:image:width" content="1146" />
<meta property="og:image:height" content="600" />
<meta property="og:image" content="_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" />

A

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📚 Documentation preview 📚: https://cpython-previews--132101.org.readthedocs.build/

[hugovk]

As #129120 (comment), we'll only generate images for the docs server, but here's indicative times, and they're very similar to before.

Read the Docs:

• Without images: 2m 18s
• With images: 3m 04s

macOS:

• Without images: 38s
• With images: 48s