docs: document public SDK APIs

[marandaneto]

💡 Motivation and Context

The SDK exposes several public methods, constructor parameters, and configuration options that were missing or had stale PHPDoc. This makes it harder for users and IDE/static analysis tools to discover the supported API surface.

This PR updates PHPDoc across the SDK to document public APIs, supported message/config shapes, feature flag parameters, request context helpers, and error tracking configuration. It also marks implementation-only classes as @internal where they are not intended as public SDK APIs, including the HTTP transport classes.

The README is also simplified to avoid duplicating Error Tracking docs and to keep repository-local setup instructions in CONTRIBUTING.md.

💚 How did you test it?

• find lib -name '*.php' -print0 | xargs -0 -n1 php -l
• php -l lib/HttpClient.php && php -l lib/HttpResponse.php
• ./vendor/bin/phpcs lib (no errors; existing line-length warnings remain)
• ./vendor/bin/phpunit --colors=never test/ (tests pass functionally; command exits 1 because of existing warnings/deprecations/no coverage driver)

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

[ioannisj]

Had a quick scan and all looks good