Docs per directive, default-theme fallback proof, v0.1.0 release

Part of #2 (Phase 1a).

Problem. The family is only usable beyond QuantEcon if a stranger can find a directive, read its options, pin the bundle and see the plain fallback work in a default theme. The lecture theme's release flow (release.yml at 47e37ed8) is the pattern: a tag that must match package.json, release notes from the changelog section, assets attached to the GitHub Release.

Proposed work.

• README per directive under docs/, each with the option table, an inline-data example, a :file: example, and a screenshot of the plain fallback in the default book theme.
• A demo project under examples/ that a reader can run with myst start.
• release.yml: tag-triggered, version guard, changelog-section guard, attaches dist/datavis.mjs; CHANGELOG.md in Keep-a-Changelog form.
• v0.1.0 cut once the eight directives and the contract are merged; v0.x thereafter tracks the report theme's needs until the contract freezes.

Acceptance criteria.

• v0.1.0 release exists with datavis.mjs attached and notes from the changelog
• Pinning that URL in a fresh mystmd project renders every example as plain tables and lists in the default theme
• Every directive has a docs page; the README links them

Start with docs/, examples/, release.yml, CHANGELOG.md, and the release workflow at 47e37ed8. Run the examples with myst start and verify a fresh mystmd project renders each directive as plain tables or lists in the default theme. Done means every directive is documented, README links the pages, and the v0.1.0 GitHub Release contains datavis.mjs with changelog notes.