Update engage-native docs for Supabase migration

[mintlify]

Summary

Updates the d-sports-engage-native docs page to reflect the Prisma-to-Supabase migration from d-sports-ecosystem/d-sports-engage-native@2da17d0.

Changes

• repositories/d-sports-engage-native.mdx:
  • Added Supabase to the tech stack table and page description
  • Added a new "Environment variables" section documenting all EXPO_PUBLIC_* vars including the new EXPO_PUBLIC_SUPABASE_URL and EXPO_PUBLIC_SUPABASE_KEY
  • Added a note clarifying that Supabase is used client-side only (not direct DB access)
  • Updated "Getting started" step 2 to link to the new env vars section

Trigger

Commit 2da17d09 in d-sports-engage-native added @supabase/supabase-js, lib/supabase.ts, and two new Supabase env vars, replacing Prisma as the data layer.