docs: add contributor onboarding guide

[kmishra1204]

Summary

Closes #8.

Adds CONTRIBUTOR_ONBOARDING.md as a practical entry point for new contributors and links it from the README.

The guide covers:

• where to start: charter, meeting minutes, issues, and PRs
• public GitHub contribution flow versus member-only meeting/channel access
• current focus-area entry points grounded in the charter and public meeting minutes
• first-time contributor checklist
• good first contribution examples
• issue pickup and document-edit guidance
• focus-area self-nomination guidance
• sensitive-data reminders for trace examples

Validation

• git diff --check
• ASCII check on CONTRIBUTOR_ONBOARDING.md
• Checked issue Create onboarding checklist and focus-group entry points #8 acceptance criteria against the added sections

Notes

PR #13 also updates repository working methods and README navigation. If it lands first, I can rebase this branch so the README link sits in the preferred Start Here section.

[kmishra1204]

Validated this current head after fetching upstream main.

Local checks:

• Branch is one commit ahead and zero behind upstream main
• git diff --check origin/main...fork/codex/contributor-onboarding-issue-8
• Changed-file scope is markdown-only: CONTRIBUTOR_ONBOARDING.md, README.md
• Relative links in changed markdown resolve inside the branch
• No TODO/FIXME/example placeholder text found in changed files