Docs System 0.12.0

Adds a2a to the product family, gives every product site a favicon and makes every build carry .nojekyll.
Consumers move their pin to this tag's commit and rebuild; no content or config change is required.

Breaking

• None.

Added

• a2a is a product: product: 'a2a' gives the A2A mark and its signature (hue #db6a98 light, #c664a8 dark; ink #8a3a64 at 7.32:1 on white, #f7c8dd at 10.95:1 on the dark surface). It joins the family strip after ESS, and a related section can name it. A new test runs the categorical checks on every signature in family order.
• favicon option of withProductSite: a path under one of the site's static directories, passed to Docusaurus as favicon. A favicon already set in the config wins.

Fixed

• A product site without a favicon serves a generated SVG at <baseUrl>b10x-favicon.svg: the mark in white on the product's ink, ringed with its hue, or on the family green without a product. Browsers had requested /favicon.ico and logged a 404.
• The build output always carries an empty .nojekyll at its root, so a GitHub Pages branch deployment does not run Jekyll, which drops every path that starts with _. A .nojekyll the site already provides is kept.

Changelog for 0.12.0

Pull request: #35