build: upgrade to Java 21 and reorganize documentation

build: upgrade to Java 21 and reorganize documentation

Bump java.version, maven.compiler source/target and the compiler
plugin configuration from 1.8 to 21.

Rename doc/ to Documentation/ following the house convention, updating
.gitignore, Tools/Update web site and AGENTS.md accordingly.

Reorganize the Documentation pages into prerequisite reading order:
the overview page leads with Introduction and Usage before
Installation and Source code; the CLI page explains class selection
concepts before the argument reference; the Java API page covers
Maven embedding before the examples.

Fix stale content: retarget sixth-3d links to the renamed Aukio 3D
project site, update CLI examples to aukio-3d artifacts and packages,
and mark the embedded-project example as outdated.

Align with the shared doc-set conventions: add the canonical
style.css, link it from every page via HTML_HEAD, give every heading
a CUSTOM_ID, use anchor-form back-links on sub-pages, and record the
conventions in AGENTS.md.