docs: data-parity guide page with screenshots

Flagged during v0.5.22 release review (PM persona).

v0.5.22 ships `data_diff` + `/data-parity` skill. The skill itself has a thoughtful 9-step workflow (`.opencode/skills/data-parity/SKILL.md`) but the docs site only has a `data_diff` section under `docs/docs/data-engineering/tools/warehouse-tools.md` (shipped in 0.5.22).

Missing: a full `docs/docs/data-engineering/guides/data-parity.md` mirroring the skill's workflow with:
- Profile-first walkthrough screenshot
- Partition output example
- Decision tree (migration validation vs query refactor)
- Algorithm selection guide
- Link from `docs/mkdocs.yml` nav

Deferred because: skill has the content; cross-linking + screenshots is a writing task, not a shipping blocker.