[prb2]

Summary

This PR supplements the existing "Nullable columns" section of the CTable Basics notebook, based on the Mask-based nullable columns for CTable doc.

Since mask-based null storage is now the default, I've added new sections 9.1 and 9.2 to cover them (shifted the existing sentinel sections). Sections 9.11 and 9.12 are also new and cover the null policies and converting between them.

This PR only partially addresses #718, I will implement the rest of it in a forthcoming PR by adding a new CTable Advanced notebook (wanted to keep the PRs small and easier to review).

Bonus: included a slice() example, the "Navigating the table" section description mentions it but didn't include a code sample.

Testing

Ran all notebook cells and confirmed correctness.

AI Tool Use

I used Claude Code (Sonnet 5.5) to generate some of the code samples:

• 9.1: generated
• 9.2 and 9.11: initially generated, but I heavily hand edited the code and wrote the explanatory comments in the output myself
• 9.12: hand wrote
• PR description: hand wrote