Docs: Replace pdoc3 with Zensical, provide a project + API documentation

pdoc3 is pretty limited and not really a state of the art tool for automated API and project documentation anymore.

So far a kind of de-facto standard for modern Python project documentation including automatic API generation has been MKDocs + Material for MKDocs + mkdocstrings for quite some years.

Zensical is the new tool that deprecates MkDocs + Material for MKDocs which will go into maintenance mode as mkdocstrings will. Zensical is developed by the same team as Material for MKDocs and mkdocstrings is and improves design wise limitations of the MKDocs based documentation tech stack.

Zensical almost reached feature parity with Material for MKDocs, supports mkdocsstrings already (until there is added a Zensical-"native" mkdocstrings replacement?) and aims at being backwards compatible.

Start by reviewing the repository's existing pdoc3 documentation setup and the linked Zensical compatibility documentation. Define how project and automatic API documentation should be provided, then verify that the completed migration produces both documentation types with the intended navigation and design.