Add extending Layout Inspector docs

Summary:
Please give feedback on the sidebar placement.

This is only the Android part. I'll put up a second diff copying the iOS thing over
but hope someone can commandeer that and check for accuracy.

Reviewed By: priteshrnandgaonkar

Differential Revision: D15181454

fbshipit-source-id: d32081feefbfb0ffc38890e835a7d5f6b78667ab

website/i18n/en.json

@@ -34,6 +34,9 @@
       "extending/js-setup": {
         "title": "JavaScript Setup"
       },
+      "extending/layout-inspector": {
+        "title": "Extending the Layout Inspector"
+      },
       "extending/new-clients": {
         "title": "Implementing a Flipper Client"
       },