docs(v1): move arithmetic-convention specs into doc/ as published rst

[FBumann]

Stacked on #717 — relocating the v1 arithmetic-convention specs into the published docs.

Note

Generated by AI (Claude Opus 4.8) per the placement decision: move the spec files into doc/, keep the scaffolding internal.

What & why. The load-bearing specs (convention.md, goals.md, legacy-removal.md) lived in arithmetics-design/, which Sphinx never rendered. This moves them into the docs site so they're discoverable, and keeps the code's §N / path references pointing somewhere real.

• Converted md → rst under doc/design/ (rst, not markdown, so no myst-parser dependency) and wired into a new "Design & Internals" toctree in doc/index.rst.
• Updated every path reference: linopy/semantics.py, the five # LEGACY: remove at 1.0 — see … comments in linopy/expressions.py, the test_legacy_violations.py header, and the scaffolding files (open-items.md, docs-plan.md) that link to them.
• release_notes.rst / migrating-to-v1.rst now use an internal :doc: cross-reference to the rendered convention page instead of a blob/master/... URL.
• The three process files (docs-plan.md, open-items.md, multiindex-feasibility.md) stay in arithmetics-design/ as internal notes, per the agreed scope.

Note: convention.md §11's MultiIndex snippet was already fixed by #856, so no content change was needed there — this PR is purely relocation + reference updates.

Verification: sphinx-build exits 0, all three pages render, no new warnings (count 891→890, just a title-underline fix); ruff clean; the convention test suite passes.

How the build was validated

The repo's docs extra pins numpy<2 while benchmarks pins 2.4.6, so a full uv sync can't resolve — validated in an isolated .[docs] env instead. Built with -D nbsphinx_execute=never (notebooks need solvers) to check toctree + :doc: resolution + rst syntax without executing notebooks.