[16.0] Fastapi: Adding clarification in documentation about 'fastify.endpoint' and it's inheritance

[TommieOlislagers]

Is your feature request related to a problem?
I just succesfully implemented the FastAPI within Odoo. One of the problems I faced was an error about inheriting the FastAPI endpoint model.

Describe the solution you'd like
I would like for the solution to be added to the documentation, as simple as it might seem.

To fix this error, you have to add FastAPI as an dependency within your custom module that inherits the fastify.endpoint.