Add docs about the GuzzleTracingMiddleware

[cleptric]

Core or SDK?

Platform/SDK

Which part? Which one?

Sentry PHP SDK

Description

We do ship a tracing middleware for the guzzle HTTP client, which was added back in 2020 https://github.com/getsentry/sentry-php/pull/1096

Suggested Solution

We should tell people how to use it in their apps.

[getsentry-release]

Routing to @getsentry/team-web-sdk-backend for triage. ⏲️

[stephanie-anderson]

@cleptric this could be closed, right?

[cleptric]

Sadly not 😢