docs: migrate documentation site to the Hextra theme

[bmorton]

Summary

Migrates the Hugo documentation site from the hugo-book theme to Hextra, installed as a Hugo Module. Delivers a modern landing page, sidebar-navigated docs, built-in full-text search, and richer shortcodes — while preserving every live URL and leaving the standalone Resource Preview WASM tool untouched.

Design spec: docs/superpowers/specs/2026-07-24-hextra-theme-transition-design.md
Implementation plan: docs/superpowers/plans/2026-07-24-hextra-theme-transition.md

What changed

• Theme via Hugo Modules — removed the hugo-book git submodule; added docs/go.mod/docs/go.sum importing github.com/imfing/hextra pinned to v0.12.3.
• Hugo bumped 0.140.2 → 0.164.0 — current Hextra (≥0.10) requires Hugo ≥0.146.0 (uses the try function). Updated in CI.
• Config — rewrote docs/hugo.toml for Hextra: navbar + GitHub link, edit-this-page, footer, theme toggle, and search enabled (FlexSearch → en.search-data.json). baseURL/title unchanged; unsafe goldmark retained; locale replaces the deprecated languageCode.
• Structure — moved all doc sections under content/docs/ behind a new Hextra landing page (content/_index.md, hero + feature cards) and a docs section root. content/preview/ stays at root.
• URL preservation — added aliases for every pre-migration URL (sections, leaf pages, /reference/ and /examples/* baked into their generators so they survive regeneration). Redirect stubs verified.
• Polish — converted note/warning blockquotes to Hextra callouts; added a self-contained button shortcode for the tools page.
• Generators & CI — repointed CRD reference + examples output to content/docs/; updated .gitignore, the verify-generated diff path, and lint globs; removed the theme submodule checkout; added a Hugo module cache (pinned actions/cache to a commit SHA).

Verification

• make preview-wasm docs-examples docs-crd-reference succeeds.
• hugo --minify builds 53 pages, zero errors (only a harmless .Site.Data deprecation WARN from Hextra's own theme code).
• Landing, docs, /preview/ (WASM intact), search index, and all 13+ alias redirect stubs present. No hugo-book references remain. markdownlint clean.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com