Replace the MkDocs site with a Fumadocs-style docs website

[Panadestein]

Reference: monoprop (Next.js + Fumadocs, MDX content under docs/content/docs/).

Our current MkDocs site is functional but plain. Move to the monoprop setup:

• Next.js + Fumadocs scaffold under docs/.
• MDX pages (getting started, concepts, features, contributing, testing, references).
• Generated Python API reference from the docstrings.
• Notebook tutorials rendered in the site.
• Update docpages.yml to build and deploy it.