Missing Explanation of headers allowlist in FastApiMCP

[sarang-pratham]

Describe

While using the library, I noticed that the FastApiMCP class supports forwarding custom HTTP headers via the headers allowlist parameter (defined in fastapi_mcp/server.py).
However, this behavior is not documented in docs/advanced/auth.mdx.

Currently the docs explain basic authorization header passthrough, but users have no indication that:

• additional headers can be forwarded
• the feature is configurable at initialization
• forwarded headers are accessible inside tools

Documenting this would make custom authentication, tracing, and metadata propagation much more discoverable.

Proposed Addition

Add a new subsection under the Advanced Auth page: Forwarding Custom HTTP Headers
explain the headers allowlist to show how to pass it to FastApiMCP(headers=[...])

Additional Context

I already prepared a pull request #248 adding this documentation.
Opening this issue to track the improvement and reference the PR.