DOC: Update thread_ts to checkpoint_id.

[gbaian10]

Issue with current documentation:

The current tutorial documents, such as the Quick Start page, still use thread_ts as an example.

However, as far as I know, it was renamed to checkpoint_id after version 0.2.0. Although the program still allows thread_ts as input, should the documents be updated to reflect the newer usage?

There might be multiple pages using thread_ts, and the ipynb outputs also need to be re-executed.

Additionally, it seems the documents don't specifically explain the purpose of checkpoint_ns. I'm curious about this field—when would we need to use it?

[vbarda]

This should be fixed now -- thanks for flagging!