docs: align platform and C API documentation

[Shinokawa]

Summary

Aligns Erika's public documentation with the current platform and C ABI surface.

• Removes stale hard-coded C ABI export counts and fixes the English header link.
• Documents HarmonyOS presenter support and the ArkTS JSON bridge in C ABI/integration guides.
• Adds a platform matrix that distinguishes compilation, CI/prebuilt coverage, and device acceptance.
• Adds release preflight validation plus SECURITY, SUPPORT, and Code of Conduct files.

Why

Several docs still described presenter support as ending at Android and stated an outdated export count, which could mislead embedders and release maintainers.

Validation

• git diff --check
• Local Markdown-link validation for all 18 changed Markdown files

No Rust, C ABI implementation, or Flutter plugin code was changed.