Standardize code blocks on fenced ```lang + markdownlint CI gate (#99)

[jonfroehlich]

Closes #99.

What

Migrates the whole textbook from Jekyll {% highlight %} Liquid tags to fenced code blocks with a required language token, and adds a CI gate so the convention stays enforced.

Content (35 pages)

• All {% highlight LANG %}…{% endhighlight %} → fenced ```lang blocks. Language tokens normalized to lowercase; cpp for all Arduino/ESP32 sketches (was C/C++); JavaScript→javascript, HTML→html, CSS→css, etc.
• Every previously-bare ``` block gets a language: text for terminal/program output, file trees, and format examples; bash for shell commands. (Satisfies MD040 site-wide.)
• Fixed two commented-out example blocks whose opener migrated but whose inline closer didn't, which left unbalanced Liquid tags. The LaTeX doc example is now correctly wrapped in raw/endraw so it renders literally.

Gate

• .markdownlint-code.jsonc — a scoped config (MD040 language-required, MD046 fenced-not-indented, MD048 backtick fences), intentionally separate from the editor .markdownlint.jsonc so the gate only checks code-block rules (not the many unrelated style nits the older content predates).
• New code-blocks job in content-lint.yml running markdownlint-cli -c (which replaces config; cli2 merges the editor config back in).
• Convention documented in website-dev.md "Code highlighting"; deliberate indented demos opt out with <!-- markdownlint-disable MD046 -->.

Why

Portability + tooling, and de-risking a future framework migration (#103) where Liquid tags would all need rewriting. {% highlight %} was legitimate when the site started — this is a forward move, not a fix.

Verification

• bundle exec jekyll build — clean (Jekyll 4).
• Scoped gate green across the repo (markdownlint-cli@0.48.0 -c .markdownlint-code.jsonc).
• Spot-checked rendered HTML: 35 files with Rouge-highlighted cpp blocks; commented blocks stay hidden; literal Liquid in the LaTeX example renders verbatim; no leaked tags.

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