Document manual session tracking

### Problem Statement

If you have a non WSGI/ASGI project than there is no automatic session tracking.
But one can easily add session tracking by hand like this:

```
from sentry_sdk.hub import Hub
from sentry_sdk.sessions import auto_session_tracking

hub = Hub(Hub.current)
with auto_session_tracking(hub):
# all the code that belong to one session goes here.
```

### Solution Brainstorm

This should be somewhere in the documentation, so that people can add session tracking to their projects.