docs: add IPythonREPL environment page

[alexzhang13]

Summary

• New /environments/ipython doc page covering both kernel modes, args, broker architecture, and the notable behavior (per-instance lock, global subcall cap, reentry guard, cell-id attribution).
• Adds IPythonREPL to the sidebar nav and the overview environments table.
• Minimal README addition: ipython in the supported-envs list/example, plus a 3-line subsection pointing at the full docs.

Test plan

• npx tsc --noEmit in docs/ passes
• Manual: visit /environments/ipython locally and verify nav + page render

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