docs: drop contents rail, cap nav at 16rem, add design rules

[bobleesj]

Restores the original widget-first docs layout and writes the rules down so they survive future reworks.

• Remove the right-hand Contents (page-toc) rail everywhere — pages showcase wide interactive widgets, so the article column gets every horizontal pixel (deletes toc-toggle.js, sets secondary_sidebar_items: []).
• Cap the left navigation at a fixed 16rem on desktop instead of scaling with the viewport, with the collapse offset kept in lockstep so the collapsed state can't shift the article off-screen.
• Add docs/maintainer/docs-design.md (in the nav under Maintainer guide) recording these rules and the verify-locally checklist.

Verified on a full local build (65 pages): nav-toggle guard green, zero horizontal overflow at 375px, sidebar expand/collapse round-trips clean at 970/1512/1859px, live Show4DSTEM widget drive on GPU.

[bobleesj]

merging. doc bug found in big screen.