Publish each model description as a web page alongside its PDF #12

The buffer-stock model description added in #5 is published two ways: as Markdown that GitHub renders, and as a PDF built with myst build --pdf. The PDF stays as the printable, citable copy. This issue is to add an HTML version on the library website, so a reader can follow links, read the equations in the browser and link straight to a section.

Questions to settle:

• One source. Build the page from the same model-description.md as the PDF so the two cannot drift apart. MyST builds both from one file.
• Where it lives. A page under docs/, such as docs/models/buffer-stock/, or a separate MyST site. The landing page is hand-written HTML with no build step, so a build step in the publish workflow is part of the decision.
• What it carries. The same title, version, date and licence as the PDF, plus the version box from Version each model description and let implementations cite the version they follow #13 and the AI-disclosure and verification indicators from Disclose AI assistance and record human verification #14. The three are planned to land together after Add buffer stock model and HARK implementation #5 merges.
• Links. The site's "description ⟨draft⟩" link, section 04 and the Contribute section would point at the page, with the PDF linked from it.
• Checks. The publish workflow's link and anchor checks should cover the built pages.

This replaces the follow-up in #5's description, "Render the model descriptions on the library website".