Send join link to the signup page

This makes it easier to onboard new members.
We'll update the landing page of the profile app to include the other info from the wiki later.

_data/menu.yml

@@ -35,7 +35,7 @@ main:
     url: /calendar
     weight: 6
   - name: Join Now
-    url: https://join.thelab.ms
+    url: https://profile.thelab.ms/signup
     weight: 7
 footer_primary:
   - name: Home