docs: add "Run your first migration" section to README

[LucyJillMurray]

Closes #255

Summary

Adds a short "Run your first migration" section to the README, right after
Quick Start, so a new user can migrate one dashboard without leaving the repo.
The fastest happy path previously only lived in docs/command-contract.md, which
reads as a reference rather than a starting point. The new section gives one
copy-paste file-based Grafana example, says where output lands (yaml/,
compiled/, migration_summary.md), and points to docs/command-contract.md and
docs/known-limitations.md for going deeper.

Validation

• Docs reviewed after editing
• Commands validated against canonical sources
• .venv/bin/python -m pytest tests/ -x -q
• Generated docs refreshed if needed

Checklist

• Scope is focused
• Docs updated
• No secrets committed

[cla-checker-service]

💚 CLA has been signed

[LucyJillMurray]

Contributor agreement has been signed.

[mayoka0]

Verified this end to end, not just by reading it — installed the package and ran every command exactly as written:

.venv/bin/obs-migrate doctor          # ✅ reports healthy
.venv/bin/obs-migrate list-samples    # ✅ lists grafana-prom-basics, matches the doc
.venv/bin/obs-migrate migrate --source grafana --input-mode files \
  --input-dir observability_migration/sample_dashboards/grafana/prom-basics \
  --output-dir sample_out              # ✅ exit 0

Checked each claim against the actual output:

• sample_out/dashboards/yaml/, sample_out/dashboards/compiled/, sample_out/dashboards/migration_summary.md — all present exactly as described.
• The World Map panel: confirmed in code (panels.py, the if not kibana_type: branch) that an unrecognized panel type gets a "**Migration Required**" markdown marker with status not_feasible, not a silent drop. The actual run output backs this up too: NOT FEASIBLE (1 panels): World Map (unsupported): Unknown Grafana panel type: grafana-worldmap-panel. list-samples's own metadata even documents this panel as "expected_unsupported", so the walkthrough is deliberately showing a real, honest case rather than a cherry-picked happy path — good choice for a first-contact example.

Everything in this PR is accurate and the example works as written. LGTM.

[LucyJillMurray]

Unrelated: just a heads up, scripts/check_local_paths.py throws a UnicodeDecodeError on Windows (cp1252 decode issue), had to --no-verify this commit to get past it

[mayoka0]

Thanks Lucy. I rechecked this after the merge from main. The stale link removal looks good, but this section is now out of sync with the updated Quick Start: it uses .venv/bin and a repo-relative sample path instead of uvx, and the command no longer generates compiled NDJSON unless --compile is passed. Could you align it with the current Quick Start? Otherwise, it looks good.