Document GitHub App + email env vars
Imported from GitHub issue El-SaMa/hollahoop#98 by @El-SaMa.
Used but not in .env.example/README/runbook: GITHUB_APP_ID, GITHUB_APP_PRIVATE_KEY, GITHUB_APP_WEBHOOK_SECRET, GITHUB_APP_SLUG, NEXT_PUBLIC_GITHUB_APP_SLUG, SENDGRID_FROM_EMAIL, SENDGRID_FROM_NAME, NEXT_PUBLIC_APP_URL (overlaps NEXT_PUBLIC_SITE_URL, hardcoded fallback), HUDDLE_WP_PLUGIN_ZIP_URL, NEXT_PUBLIC_HUDDLE_WP_PLUGIN_DOCS_URL, NEXT_PUBLIC_ANALYTICS_SRC/DOMAIN. .env.example has ~25 vars; rest only in PRODUCTION_RUNBOOK.md.