Standardize code-block formatting and enforce it in CI

[jonfroehlich]

Problem

Code blocks across the textbook are inconsistent in both delimiter style and language tokens, which hurts rendering consistency, the copy-code button, and maintainability. Two delimiter systems coexist:

• {% highlight %} Liquid tags (older revised modules: arduino, communication, advancedio)
• Fenced blocks (newer esp32 work — uniformlycpp)

Evidence (counts across *.md, excluding _site/)

Same language, multiple tokens — Arduino/ESP32 sketches (all C++) appear as:

• {% highlight C %} ×52, {% highlight C++ %} ×59, {% highlight cpp %} ×26, plus ~80 fenced cpp and a couple C fences — 3–4 tokens for one language
• HTML ×14 vs html ×1

Latent bug: nearly every block closes with {% endhighlight C %} / {% endhighlight JavaScript %} etc. Jekyll's endhighlight takes no argument — tolerated today but non-standard/fragile.

Bare fences with no language (no syntax highlighting): communication/serial-intro.md (~10), plus serial-print, rgb-led-fade, led-blink3, inside-arduino, esp32-ide, advancedio/oled, and the signals pages.

Target convention already exists: the esp32 module converged on ```cpp.

Proposed standard

1. Fenced ``` blocks with a required language token (standard Markdown, native Rouge highlighting, copy-button friendly, avoids the endhighlight footgun). Migrate all `{% highlight %}` → fenced.
2. One token per language: cpp for all Arduino/ESP32 sketches (Rouge has no arduino lexer; cpp is correct/supported), plus javascript, python, html, css, json, bash.

Enforce in the build/test system

• markdownlint built-ins: enable MD040 (fenced blocks must declare a language), MD046 (fenced, not indented), MD048 (``` not ~~~).
• Custom scripts/lint_code_blocks.py for what markdownlint can't see: remaining {% highlight %} tags, variant/wrong language tokens (C/C++ → cpp), and the endhighlight LANG form.
• Wire both into CI (depends on Evaluate moving to a GitHub Actions Jekyll deploy (enabler for V2.0) #98) so PRs fail on violations.
• Document the convention in website-dev.md.

Tasks

• Decide final delimiter + token table
• Migrate {% highlight %} → fenced; normalize language tokens; remove bare fences
• Add markdownlint rules MD040/MD046/MD048
• Write scripts/lint_code_blocks.py
• Add CI gate (see Evaluate moving to a GitHub Actions Jekyll deploy (enabler for V2.0) #98)
• Document in website-dev.md

Related: CI gate depends on the GitHub Actions pipeline in #98.