Document the ReScript 13 alpha contract and safe contributor workflow #393

Part of #382 — v1.0.0 alpha readiness.

Problem

The project intentionally supports ReScript 13 alpha, but docs/content/docs/philosophy.mdx:9-L11 still describes ReScript 12. The README/status pages do not define the planned 1.0.0-alpha.N compatibility contract, and the contributor update recipe includes destructive git clean -xdf: docs/content/docs/contributing/getting-started.mdx:34-L50.

Acceptance criteria

• Document the tested compiler alpha version(s), required configuration, normal install command, and a minimal working consumer example; coordinate with the package peer-range issue.
• State that library alpha releases may continue alongside ReScript 13 alpha and may contain breaking changes between alphas.
• Distinguish package/compiler compatibility from browser API availability; describe tested environments and remaining known limitations without claiming exhaustive browser support.
• Replace the routine git clean -xdf update recipe with a non-destructive workflow that preserves local work. Do not advise destructive cleanup as a prerequisite for updating a fork.
• Refresh project status and contributor architecture/testing examples to match the current flat public API and actual test commands.
• Add initial changelog/migration notes and link the alpha release checklist; avoid duplicating version/channel policy independently of the release workflow.
• Review existing issues Rename overloaded constructor bindings from numbered make* names to semantic from* names #236, Simplify package module/type structure #237, Improve generated overloads #27, API overview #23, How to model Errors #4, and Consider adding useful extension module #5 for completed, remaining, or deliberately deferred work; record dispositions without implying all historical requests block alpha.
• Docs build, formatting, and supported consumer examples pass.

The 80 private-record TODOs, generator reproducibility, and a full browser matrix may remain documented follow-ups; do not make a stable ReScript compiler an alpha prerequisite.

Coordinate compiler requirements with #384 and release/version instructions with #385.