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