Document icon for data entry sections (#2292)

* Document icon for data entry sections

* Try to find reason for build error

* Update data_entry_flow_index.md

* tiny tweak

---------

Co-authored-by: c0ffeeca7 <38767475+c0ffeeca7@users.noreply.github.com>

docs/data_entry_flow_index.md

@@ -153,9 +153,12 @@ class ExampleConfigFlow(data_entry_flow.FlowHandler):
 
 #### Grouping of input fields
 
-As shown in the example above, input fields can be visually grouped in sections. Grouping input fields by sections influence
+As shown in the example above, input fields can be visually grouped in sections. 
-both how the inputs are displayed to the user and how user input is structured. In the example above, user input will be
+
-structured like this:
+Each section has a [translatable name and description](#labels--descriptions), and it's also possible to specify an icon.
+
+Grouping input fields by sections influences both how the inputs are displayed to the user and how user input is structured.
+In the example above, user input will be structured like this:
 
 ```python
 {
@@ -170,6 +173,22 @@ structured like this:
 
 Only a single level of sections is allowed; it's not possible to have sections inside a section.
 
+To specify an icon for a section, update `icons.json` according to this example:
+
+```json
+{
+  "config": {
+    "step": {
+      "user": {
+        "sections": {
+          "ssl_options": "mdi:lock"
+        }
+      }
+    }
+  }
+}
+```
+
 #### Labels & descriptions
 
 Translations for the form are added to `strings.json` in a key for the `step_id`. That object may contain the folowing keys: