Issue #3609280: Add the project logo and favicon to the documentation site

Wires the project logo and favicon into the MkDocs Material theme so the generated documentation site shows the Yoyaku branding.

Adds logo.png / logo.svg at the module root and docs/logo.png / docs/favicon.png, and sets in mkdocs.yml:

theme:
  name: material
  logo: logo.png
  favicon: favicon.png

Paths resolve relative to docs_dir (docs/). The docs site is generated from the default branch (1.x).