chore!: require Java 21, drop servlet-api and CommonPathResolver

chore!: require Java 21, drop servlet-api and CommonPathResolver

Build: compiler source/target 8 to 21 (Debian 13 ships OpenJDK 21
and 25); remove the unused provided-scope servlet-api 2.5 dependency,
which nothing in the tree ever imported.

API: delete CommonPathResolver, a trivial System.getProperty
("user.home") wrapper; package-info and docs updated accordingly.

Documentation: doc/ renamed to Documentation/ per house convention.
The page gains stable CUSTOM_ID anchors on every heading and the
shared style.css replaces the inline style block; the Maven
quick-start section moves ahead of the feature tour, feature
subsections follow a familiarity ramp, and a new building-from-source
section documents the Debian 13 JDK packages and the Maven build.