Add callout about trace decorator

[matejminar]

Problem: By default, Sentry will trace well known operations like HTTP calls and database queries, but not for pieces of code that perform some other operations. This isn't clear today in docs or the setup guides for tracking.

Proposed solution:

• Update docs to be more explicit:
  The Sentry SDK for Python does a very good job of auto instrumenting your application. If you use one of the popular frameworks, we've got you covered because everything is instrumented out of the box. The Sentry SDK will check your installed Python packages and auto enable the matching SDK integrations.

• add a callout in the setup instructions:
  "By default, Sentry will trace well known operations like HTTP calls and database queries. If you want to enable tracing in a piece of code that performs some other operations, add the @sentry_sdk.trace decorator"

• let's verify this makes sense for other platforms too (e.g. JS)