Add Q&A 1 slide deck + drop course page ToC

[natolambert]

Summary

• New teach/course/qa-01.md — first round of a Q&A slide deck series. Each slide is a placeholder: reader's question quoted, light context in the body, attribution in the footer. I'll talk through each one live and we can flesh out follow-up slides after.
• Uses the title-banner layout on the title slide to make Q&A decks visually distinct from the lecture decks.
• Drops the small Contents mini-ToC from book/templates/course.html (the page is short enough without it) and adds the Q&A deck as an inline entry in the Lectures list, after Lecture 4.

Questions in this round

Lecture 2 — IFT, Reward Models, & Rejection Sampling

• One model vs. many for synthetic SFT data? — @mufeezamjad790 (YouTube)
• Should teacher distribution match the base model? — @mufeezamjad790 (YouTube)
• Unified RM benchmark across RM types on one dataset? — @jeromeeusebius (YouTube)
• How important is balancing domain proportions in an SFT mix? — @jeromeeusebius (YouTube)

Lecture 3 — RL Motivation & Math

• What is the shape of $\rho$? — @JGLambourne (YouTube)
• Is GAE the only way to compute the advantage in PPO? — #382
• Should the notation table define $T$, $K$, $G$? — #382
• Are slides 59–61 in Lec 3 in the right order? — #382

Test plan

• Check the rendered course page on the Pages preview: Contents nav is gone, Q&A entry shows up after Lecture 4 with PDF / Slides / Source buttons.
• Confirm the Q&A deck builds (HTML + PDF) and the title-banner layout looks right.
• Click through each Q slide and sanity-check the per-slide footer attribution renders.

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