docs: present compatibility to OverPy developers in plain language

[Teakowa]

Closes #22

Summary

Rework the human-facing compatibility presentation in the README so an OverPy developer can see current support without learning WrightKit's internal state taxonomy.

Changes

• Replaced the compatibility table (Capability | Matrix scope | State) and the internal state legend with a concise capability table using only ✅ Supported / 🟡 Partial / ⏳ Not yet, described in OverPy terms (core syntax, declarations, preprocessing, JavaScript macros, rules, builtins, receiver/member functions, enums, switch/string modifiers, advanced directives, OPY→Workshop, Workshop→OPY).
• Removed Matrix scope and the internal state vocabulary (frontend-supported, semantic-supported, lowering-dependent, …) from the README body; the machine-readable matrix and per-feature evidence remain in docs/opy/support-matrix.md + compatibility/support-matrix.json and are linked from the compatibility section.
• Reworded the intro to present opy-rs as WrightKit's standalone Rust implementation of OverPy, and trimmed internal terms (Opy HIR v1, manifest-declared) from the primary path.
• No support-state data, corpus, or compatibility claims changed.

Validation

• No internal support-state vocabulary or Matrix scope remains in the README body.
• All relative links resolve; 0 em/en dashes.
• Each table row maps to the existing matrix states without changing claims (✅ = frontend/semantic-supported, 🟡 = declared subset supported with full surface planned, ⏳ = planned or workshop-rs-dependent).