Docs Doesn't Respect Base URL

[awickham10]

The /docs page doesn't respect the FastA2A url parameter, resulting in 404s and missing agent card when running on a URL that is not at the root.

✅ http://localhost:8000
❌ http://localhost:8000/my-agent

[awickham10]

I think the docs.html could be converted to a Jinja template so the URL could be injected. I'd be happy to open a PR if you'd like.