TOC: introduce "Markov Decision Processes" section

[mmcky]

Summary

Implements the lecture reorder agreed in #4 and finalised in this comment thread:

• Introduction: drops discrete_dp → now contains only short_path (1 lecture).
• Job Search: drops mccall_q → now 8 lectures.
• NEW section "Markov Decision Processes" between Income Fluctuation Problems and LQ Control:
  • discrete_dp (moved from Introduction)
  • inventory_q (added in SYNC: add inventory_q.md from lecture-python.myst #19)
  • rs_inventory_q (added in SYNC: add rs_inventory_q.md from lecture-python.myst #20)
  • mccall_q (moved from Job Search)

This is the third (and final) PR for #4. PRs #19 and #20 added the two new files to the repo but kept them out of the build (only_build_toc_files: true); this PR activates them and closes the reorder.

Why this structure

discrete_dp is the MDP-theoretical foundation that the three Q-learning lectures build on, so grouping them together gives a coherent reading order and fixes the difficulty cliff that motivated #4 (per @jstac's note that discrete_dp was a lot more advanced than the introductory lectures that followed it).

Test plan

• CI build succeeds (this is the first build with inventory_q / rs_inventory_q in the TOC).
• Generated HTML shows the new section between Income Fluctuation Problems and LQ Control.
• The 10 {doc} references to inventory_q inside rs_inventory_q.md resolve.
• The {cite}\Sargent_Stachurski_2025`reference ininventory_q.md` resolves (entry added in SYNC: add inventory_q.md from lecture-python.myst #19).
• No internal {doc} references break for discrete_dp or mccall_q after the move (verified locally — no cross-references in the repo depended on their old paths).

Follow-up

Update #7 to bump the lecture-python.myst mapping from 25 → 27 files.

Closes #4

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the Jupyter Book table of contents to implement the agreed lecture reordering by introducing a new “Markov Decision Processes” section and relocating existing lectures to match the intended learning progression.

Changes:

• Remove discrete_dp from Introduction (leaving short_path as the sole lecture there).
• Remove mccall_q from Job Search.
• Add a new Markov Decision Processes part containing discrete_dp, inventory_q, rs_inventory_q, and mccall_q between Income Fluctuation Problems and LQ Control.

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