Add global setting and default for discovery relays

app/views/admin/settings/services/_nostr.html.erb

@@ -31,13 +31,28 @@
       ) %>
 </ul>
 </section>
+
 <section>
 <h3>Zaps</h3>
 <ul role="list">
   <%= render FormElements::FieldsetResettableSettingComponent.new(
         key: :nostr_zaps_relay_limit,
         title: "Relay limit",
-        description: "The maximum number of relays to publish zap receipts to"
+        description: "The maximum number of sender-defined relays to try to publish zap receipts to"
       ) %>
 </ul>
+</section>
+
+<section>
+<h3>Onboarding</h3>
+<ul role="list">
+  <%= render FormElements::FieldsetComponent.new(
+        title: "Discovery relays",
+        description: "Used to discover a user's published relay list and/or profile"
+      ) do %>
+    <%= f.text_area :nostr_discovery_relays,
+      value: Setting.nostr_discovery_relays.join("\n"),
+      class: "h-44 w-80" %>
+  <% end %>
+</ul>
 <% end %>