Replace Docusaurus docs with Astro website

Ref: spec-untp#641

This PR completely replaces the current documentation website with a new Fora website build with Astro.

IMPORTANT: The new website fetches events from the AddEvent API and requires two environment variables to be configured in the CI/CD pipeline. The build will not fail if these variables are missing, but no events will be fetched, so the events list will be empty.