Two explainers still say something is "coming soon" that already exists #1128

Where: explainers/sampling-bias.md (line 231) and explainers/proxy-variables.md (line 160).

Current gap: sampling-bias.md says "Demographic parity (coming soon) measures whether positive prediction rates are equal across groups", although explainers/demographic-parity.md has been published for a long time and the sentence should link to it. proxy-variables.md ends a list with "Coming soon: Facial recognition bias, HMDA loan bias, healthcare AI", which is stale (three healthcare explainers and the Healthcare Readmission audit exist; the two others are still listed as open audits in the ROADMAP).

Repro: grep -n -i "coming soon" explainers/*.md

Suggested approach: link demographic parity in sampling-bias.md and drop the "(coming soon)" wording; in proxy-variables.md replace the line with links to the healthcare explainers and audits that exist, and say that facial recognition and HMDA are planned in the ROADMAP. Then rebuild.

Done when: grep -i "coming soon" explainers/*.md returns nothing, the sentences link to the pages they mention, and the generated pages are rebuilt.