docs: add documentation for experimental browser proxy handler

[AlanPonnachan]

Summary

This PR adds the documentation for the newly merged experimental browser proxy handler (logfire.experimental.forwarding.logfire_proxy) introduced in #1697.

It bridges the gap between frontend browser telemetry and Python backends by explaining how to securely route OTLP payloads to Logfire without exposing the Write Token in the client-side code.

Changes Made

• docs/integrations/javascript/browser.md:
  • Updated the "Securely Sending Traces" admonition to explicitly mention the new Python built-in handler.
  • Added cross-reference links to the FastAPI and Starlette documentation.
  • Updated the TS example to show url.pathname = "/logfire-proxy/v1/traces".
• docs/integrations/web-frameworks/fastapi.md:
  • Added a "Proxying Browser Telemetry" section.
  • Provided idiomatic FastAPI examples showing how to mount the handler using @app.post while injecting the Request object.
  • Included an example demonstrating how to protect the unauthenticated endpoint using FastAPI's Depends (addressing security considerations discussed in the previous PR).
• docs/integrations/web-frameworks/starlette.md:
  • Added a "Proxying Browser Telemetry" section.
  • Provided an example of mounting the handler directly as a Starlette Route.
  • Added a security admonition regarding authentication and rate-limiting for production use.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[alexmojaki]

Thanks!