website: add Chat Relay guide to the docs navbar (#7271)

Adds a 'Host Chat Relay' entry to the Reference dropdown, after 'Host
XFTP Server', linking to /docs/chat-relay.html.

website/langs/en.json

@@ -242,6 +242,7 @@
     "docs-dropdown-12": "Security",
     "docs-dropdown-14": "SimpleX for business",
     "docs-dropdown-15": "Verify & reproduce builds",
+    "docs-dropdown-16": "Host Chat Relay",
     "newer-version-of-eng-msg": "There is a newer version of this page in English.",
     "click-to-see": "Click to see",
     "menu": "Menu",

website/src/_data/docs_dropdown.json

@@ -24,6 +24,10 @@
             "title": "docs-dropdown-5",
             "url": "/docs/xftp-server.html"
         },
+        {
+            "title": "docs-dropdown-16",
+            "url": "/docs/chat-relay.html"
+        },
         {
             "title": "docs-dropdown-6",
             "url": "/docs/webrtc.html"