docs: streamline README and centralize documentation index

[Teakowa]

Summary

Streamline the public README after the human-facing compatibility cleanup and make docs/README.md the single documentation index.

Changes

• Rework the README around product capabilities, CLI/library entry points, compatibility, validation, contributing, and license.
• Remove the repeated Stable contracts and Current limitations prose where the compatibility table already communicates the user-visible boundary.
• Keep the no-WrightKit-only-dialect rule visible as a GitHub IMPORTANT alert.
• Use inline links for architecture, tooling, source editing, evidence, and detailed compatibility references.
• Move pinned-reference detail into a footnote rather than the main reading flow.
• Add docs/README.md as the canonical index for architecture, APIs, compatibility evidence, HIR, provenance, and maintainer references.

Impact

A new OverPy developer gets the project overview and current support directly from the root README, while maintainers still have direct access to the existing detailed contracts and machine-readable evidence through the docs index.

No compatibility implementation, support-matrix state, corpus evidence, or repository ownership boundary is changed.

Validation

• Branch is based on the current main and is not behind it.
• README relative links were checked against the current repository tree.
• docs/README.md links only to existing documentation/evidence paths.
• Documentation-only change; no code or machine-readable compatibility data changed.