Fix broken link (#2706)

[Infamous003]

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[pushfoo]

Thank you! This is some very good attention to detail! 👀

For future readers

We're linking the element ID in the generated HTML because:

1. Sphinx has a long-standing issue with :target: as a reference
   • See sphinx-doc/sphinx#4351)
   • Maybe it will be fixed one day
   • Don't expect it any time soon
2. We need the thumbnail to be clickable now
3. Some example pages may end up holding multiple headings
4. Sphinx uses <span id="shapes-slow"></span> and similar as link headings
5. Adding #shapes-slow ID targets the specific one in question in a somewhat future-proof way

A better future solution would help, but this is excellent work!