doc: add PR Reviewer Guide + visibility wiring (uniform with MobilityDB/MobilitySpark)

[estebanzimanyi]

Summary

Adds doc/contributing/reviewer-guide.md for MobilityDuck, structurally identical to the MobilityDB and MobilitySpark guides at the same path:

• Canonical sections: How to find this guide → CI legend (5 symbols incl. ⚠️) → Dependency chains → Tier 1/2/3 → Review checklist
• Repo-specific content: 19 open PRs, the four correctness-fix prerequisites (fix(bindings): fix SpanSet serialization size and floatspan distance datum #106/fix(bindings): fix distance return types; add + shift alias for tstzset/tstzspan+interval #107/fix: splitNspans / splitEachNspans on spanset return SpanSet not LIST<Span> #58/feat(bindings): set(LIST(GEOMETRY), INTEGER) overload — explicit SRID #61), the consolidate/ parity batch (feat(bindings): temporal ops parity — boxops / comparison / position / precision / same #97–docs: CONTRIBUTING.md + PARITY.md user guide + PARITY-INVENTORY.md #105), and the recent feature additions (feat(parity): unskip tnumber math, tbox, and similarity parity tests #108–feat(geo): add nearestApproachDistance, expandSpace, && for TGEOMPOINT #114)

Wires visibility so reviewers landing on the repo find the guide quickly:

• .github/PULL_REQUEST_TEMPLATE.md — every new PR description starts with a banner pointing to the guide
• README.md — new "For contributors and reviewers" section right before the Requirements section

The guide is updated in the same commit as any PR queue change (one-liner status change is enough; a fuller rewrite is needed when the dependency graph changes).

Cross-repo coordination

Companion PRs landing the same uniformization + visibility wiring:

• MobilityDB: PR #931
• MobilitySpark: PR #8

Once all three merge, anyone landing in any of the three platform repos finds the same canonical guide at the same path.

Test plan

• Render the guide on GitHub — tables and dependency chains display correctly
• After merge, open a draft PR — verify the template banner appears
• After merge, view the README on the repo home page — verify the "For contributors and reviewers" section appears