docs: show phone-sized diagrams on phones

[EivMeyer]

Summary

On a phone the documentation now shows each diagram's phone-sized drawing in place of the wide one. Before, the wide drawing shrank to the column until its labels were unreadable, with a link to the phone-sized file underneath.

Details

• scripts/docs_theme.py turns a diagram image followed by a link to its -mobile.svg into one <picture> that switches at 760px, the site's phone breakpoint. A page without that pair renders as before.
• The README is unchanged, so GitHub keeps the wide drawing and the link.
• check_links in scripts/build-docs.py now reads srcset, so a missing drawing still fails the build. That also covers the hero image's sources.

Verification

• scripts/build-docs.sh, and test_docs_versions.py (23 pass)
• A page whose srcset names a missing file fails the link check
• Chrome, Firefox and WebKit at 1440, 700 and 390px: the wide drawing shows on desktop, the phone-sized one below 760px, with no horizontal overflow
• opcore run pre-commit: Verify clean, no Sense findings; it exits requires_attention on the repository's partial Sense coverage, as on main

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