Sphinx docs and gym README

[elprofesoriqo]

Summary

• Replace MkDocs with Sphinx + Furo (FlowEdge parchment/wine, Excalidraw figures).
• Rewrite README and docs around the cybersecurity RL gym: SIEM vs oracle, first run, Arena, packs.
• No Light/Dark/Auto picker; theme follows the browser. Keep arXiv citation and CybORG/DSTG license notes.

Test plan

• pip install -e ".[docs]"
• sphinx-build -b html -W --keep-going docs docs/_build/html
• Open docs/_build/html/index.html and confirm nav has no Light/Dark/Auto controls

Checklist

• One issue, one PR
• Tests added or updated
• Did not vendor a training algorithm (MAPPO / QMIX / CT-GMARL stay out of tree)
• User-path docs updated only if the user path changed

Stacked on #9. Retarget to main after #9 merges.

[elprofesoriqo]

Reviewed the Sphinx/README stack (5 commits). Merge after #9; retarget to main then.

Looks good

• MkDocs is gone; Sphinx -W is the documented gate.
• Light / Dark / Auto controls are removed from the custom nav, and Furo's theme toggle is hidden. Browser color-scheme still applies.
• README keeps the arXiv citation from main and the CybORG/DSTG license note.

Notes (non-blocking)

• DATASHEET.md exists at repo root and as docs/DATASHEET.md. Fine if intentional; otherwise pick one.
• README already mentions docs.yml / GitHub Pages. That workflow arrives in #11.