Add automated contributor recognition in README and docs

[saksham2001]

Summary

Auto-generates a contributor avatar grid (photo + username) from the GitHub API.

• README → iOS contributors only (saksham2001/PulseLoopiOS)
• New docs page → iOS + Android merged & deduped (+ foureight84/PulseLoopAndroid)
• Weekly + manual GitHub Actions workflow; commits only on change

Type of change

• ✨ New feature (non-breaking change that adds functionality)
• 📝 Docs only

How was this tested?

• Ran scripts/update_contributors.py locally against the live API — grids populate correctly (README 4, docs 6)
• N/A (docs / non-code change — no device needed)

Privacy & data

• This change does not send health data off-device without explicit user action.
• No secrets, API keys, or personal data are committed.

Checklist

• I updated docs / README where relevant.
• I read the Contributing guide.

Pointers

• scripts/update_contributors.py — stdlib-only generator, marker-based injection
• .github/workflows/contributors.yml — weekly cron + workflow_dispatch
• docs/project/contributors.md + mkdocs.yml nav entry