docs: separate dependencies from main pouch deps

After discussion at https://github.com/apache/pouchdb/issues/9135, the website build was moved from Ruby to Javascript in https://github.com/apache/pouchdb/pull/9146.

This change also mixed the website dependencies with those of pouchdb itself.

This means that in future there is risk that stagnation of the docs build will hold back dependencies of the main pouchdb code. This should be avoided.

A simple approach would be to declare a separate package.json in the docs directory, completely independent of the package.json in the project root.