Documentation: add explainer for FastAPI

> Awesome thanks. @omsawant-coder how about writing up a documentation section on https://hamilton.dagworks.io/en/latest/how-tos/microservice/ that uses the example linked?
> So task would be:
> * Provide an explanation of how you could use Hamilton in a FastAPI webservice using Asynchronous python, using the example code we already have.

_Originally posted by @skrawcz in [#1186](https://github.com/DAGWorks-Inc/hamilton/issues/1186#issuecomment-2799073187)_

Start with the microservice documentation at docs/how-tos/microservice/ and the existing example code referenced in the issue. Explain how to use Hamilton in a FastAPI web service with asynchronous Python, and consider the work done when the page clearly covers that example and workflow.

Written by the indexing model from the issue text.