Add documentation about tracing

[VincentLanglet]

Issue

This an issue with:
https://docs.sentry.io/platforms/php/guides/symfony/

Description

Tracing support is new in sentry-symfony
getsentry/sentry-symfony#477
getsentry/sentry-symfony#426
getsentry/sentry-symfony#460
getsentry/sentry-symfony#430
getsentry/sentry-symfony#423

The doc should be updated to explain how to use it.

[rhcarvalho]

Noting there's already https://docs.sentry.io/platforms/php/guides/symfony/performance/.