disables signups via env variable (#6212)

* disables signups via env variable

* Apply suggestions from code review

Co-authored-by: Peer Richelsen <peeroke@gmail.com>
Co-authored-by: Omar López <zomars@me.com>

.env.example

@@ -152,3 +152,6 @@ SENTRY_IGNORE_API_RESOLUTION_ERROR=
 NEXT_PUBLIC_APP_NAME="Cal.com"
 NEXT_PUBLIC_SUPPORT_MAIL_ADDRESS="help@cal.com"
 NEXT_PUBLIC_COMPANY_NAME="Cal.com, Inc."
+# Set this to true in to disable new signups
+# NEXT_PUBLIC_DISABLE_SIGNUP=true
+NEXT_PUBLIC_DISABLE_SIGNUP=