docs: add per-module READMEs

[DemchaAV]

Why

The 2.0 split produced eleven module directories, but only examples/ and benchmarks/
had a README. A developer browsing render-pdf/, templates/, bundle/, etc. on GitHub had
no per-folder explanation of what the artifact is or when to depend on it.

What

A README in each remaining module directory:

• Published: render-pdf, render-docx, render-pptx, templates, testing, the
  graph-compose wrapper, bundle, fonts, emoji — each states what the artifact is, when
  to depend on it versus the alternatives, its key entry point (with a real usage snippet), and
  links back to the root install matrix + the modules migration guide.
• Internal: qa, aggregator — marked "not published to Maven Central", with the reactor
  build command.

Coordinates are stated version-free (each links the root install matrix for the pinned
snippet), so cut-release.ps1 never has to touch these files and they can't drift.

Tests

• All relative links across the eleven files resolve (link-checker).
• A cold fact-check verified every claim against the real poms/source — coordinates, packaging
  (jar wrapper / pom bundle), opt-in/bundling, the SPI registration, and each code snippet;
  it caught three snippet signature errors (compose(session, spec), assertMatchesBaseline,
  the instance RichText.text("").emoji(...)) which are fixed.