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.

首先找到現有的工作階段追蹤文件，並根據文件化的 API 檢查 issue 中的 Python 範例。加入使用 Hub 和 auto_session_tracking 手動包裝非 WSGI/ASGI 工作階段的指南；當使用者能夠找到並依照適用於其專案類型的範例操作時，即表示完成。

由索引模型根據 Issue 內容生成。