docs(T10): fast-follow backlog (tracking)

## Parent
#8

## What to build
A single **tracking** ticket capturing post-talk docs work so nothing is dropped when talk-day scope is cut. Not agent-grabbable as one unit — each item below is split into its own ticket when picked up.

## Backlog (fast-follow)
- [ ] Tutorial 3 (nb02, tuning-gated; frozen outputs with an nbstripout carve-out).
- [ ] Remaining how-tos: hierarchical models, experimental conditions, serialization, convenience helpers, **tuning (`tune_*`)**, "Validate before MCMC".
- [ ] Estimator-chooser page.
- [ ] sbi-migration guide.
- [ ] Shapes Explanation essay.
- [ ] DDM hierarchical case study.
- [ ] pydata version switcher once ≥2 releases exist.

## Note
Blocked by the full talk-day set (T1–T9). Tracking issue; not labelled `ready-for-agent`.

## Blocked by
- #9 · #10 · #11 · #12 · #13 · #14 · #15 · #16 · #17

This is a tracking issue for multiple documentation tasks. Each item in the backlog should be split into its own ticket when work begins. The work is blocked by completion of T1–T9. To start, review the linked parent issue #8 and the blocked issues #9–#17 to understand the context and dependencies.

Written by the indexing model from the issue text.