Add developer notes explaining documentation architecture

[basnijholt]

Summary

Adds documentation for future maintainers explaining the documentation build system and why it's designed the way it is.

The Recursion Problem

The docs use markdown-code-runner to generate content, but also contain examples of the tool's syntax. If those examples were executed, they'd become useless as examples - users need to see the template syntax, not executed results.

What This Documents

• CODE:SKIP directive - Prevents example code blocks from being executed
• PLACEHOLDER pattern - OUTPUT sections contain placeholder text that gets replaced during CI, but generated content is never committed
• Nested HTML comment escaping - How to write code that references markers inside HTML comments without breaking parsing
• Pre-commit hook - The hook that prevents accidental commits of generated content
• Why we don't commit generated output - Copyability, recursion avoidance, single source of truth, cleaner diffs

Test plan

• File is well-formatted markdown
• Pre-commit hooks pass