text-demo: Add text-specific screenshot. Add text demo to _book (#4939)

Author: bileschi

docs/_book.yaml

@@ -21,6 +21,8 @@ upper_tabs:
         path: /tensorboard/image_summaries
       - title: "Model graphs"
         path: /tensorboard/graphs
+      - title: "Text data"
+        path: /tensorboard/text_summaries
       - title: "Hyperparameter tuning"
         path: /tensorboard/hyperparameter_tuning_with_hparams
       - title: "Embedding projector"

docs/images/text_simple.png

@@ -192,7 +192,7 @@
         "id": "c8n8YqGlT3-c"
       },
       "source": [
-        "<!-- <img class=\"tfo-display-only-on-site\" src=\"https://github.com/tensorflow/tensorboard/blob/master/docs/images/images_single.png?raw=1\"/> -->"
+        "<!-- <img class=\"tfo-display-only-on-site\" src=\"https://github.com/tensorflow/tensorboard/blob/master/docs/images/text_simple.png?raw=1\"/> -->"
       ]
     },
     {