docs: clarify newcomer contribution paths and checks

[LunaStev]

Summary

• Help newcomers choose a small contribution and find the right place for questions.
• Document proportionate checks for prose, examples, compiler changes, and Python tooling.
• Link current CI sources and consolidate README verification guidance.

Validation

• Passed git diff --check.
• Rendered CONTRIBUTING.md and README.md with Pandoc; validated all 51 relative links and rendered Markdown anchors.
• Ran the documented CI-plan preview successfully, returning 12 stages without provisioning or executing them.
• Confirmed the good-first-issue label through GitHub's public API.
• Compiler builds and runtime tests were not run because only documentation changed.

Motivation

Make the first contribution easier to scope and verify, especially for documentation-only work.

Target and compatibility impact

None. These changes only update CONTRIBUTING.md and README.md; no compiler, standard-library, CLI, or ABI behavior changes.

Checklist

• The commit includes a DCO Signed-off-by line.
• Documentation-only changes are covered by Markdown/link checks and the documented CI-plan preview.
• User-facing contribution guidance is updated.
• The compiler and std/ license boundary is unchanged.