feat: make build-fast target (PDF only)

[adamdaw]

Add a fast-build target that generates only PDF, skipping HTML and DOCX.

Context

`make all` generates PDF + HTML + DOCX (~3-4 minutes). During iterative authoring, most users only need PDF. A `make build-fast` or simply using the existing `make build` is sufficient, but the naming could be clearer.

Acceptance Criteria

• Clear documentation that `make build` = PDF only, `make all` = PDF+HTML+DOCX
• Consider `make pdf` as alias for `make build` for clarity

Reference: GNU Make conventions