doc: Update new doc links to add-on (#118)

Update new doc links to the following
add-ons:
* Serial Modem
* Hello nRF Cloud
* Asset-Tracker-template

Signed-off-by: divya pillai <divya.pillai@nordicsemi.no>

Author: divipillai

index/hello-nrfcloud.json

@@ -18,7 +18,7 @@
                     "sdk": "v2.8.0-rc2"
                 }
             ],
-            "docsUrl": "https://docs.nordicsemi.com/bundle/addon-nrfcloud_firmware-latest/page/index.html"
+            "docsUrl": "https://nrfconnectdocs.nordicsemi.com/addons/addon-nrfcloud_firmware/latest/index.html"
         }
     ]
 }

index/nrfconnect.json

@@ -238,7 +238,7 @@
       "tags": ["lte", "dfu", "connectivity"],
       "apps": "app",
       "avatar": "https://avatars.githubusercontent.com/u/40860733?s=200&v=4",
-      "docsUrl": "https://docs.nordicsemi.com/bundle/asset-tracker-template-latest/page/index.html",
+      "docsUrl": "https://nordic-semiconductor.fluidtopics.net/r/bundle/Asset-Tracker-Template/page/Asset-Tracker-Template",
       "releases": [
         {
           "tag": "placeholder",
@@ -289,7 +289,7 @@
       "tags": ["lte", "connectivity"],
       "apps": "app",
       "avatar": "https://avatars.githubusercontent.com/u/40860733?s=200&v=4",
-      "docsUrl": "https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/index.html",
+      "docsUrl": "https://nrfconnectdocs.nordicsemi.com/addons/addon-serial_modem/latest/index.html",
       "releases": [
         {
           "tag": "v2.0.0-preview1",