docs: dt: writing-schema: Include the example schema in the doc build

The example-schema.yaml file serves as documentation, so let's include it
from writing-schema.rst.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-4-robh@kernel.org

Documentation/devicetree/writing-schema.rst

@@ -8,6 +8,8 @@ written in a JSON compatible subset of YAML. YAML is used instead of JSON as it
 is considered more human readable and has some advantages such as allowing
 comments (Prefixed with '#').
 
+Also see :ref:`example-schema`.
+
 Schema Contents
 ---------------
 
@@ -170,3 +172,12 @@ json-schema Resources
 `JSON-Schema Specifications <http://json-schema.org/>`_
 
 `Using JSON Schema Book <http://usingjsonschema.com/>`_
+
+.. _example-schema:
+
+Annotated Example Schema
+------------------------
+
+Also available as a separate file: :download:`example-schema.yaml`
+
+.. literalinclude:: example-schema.yaml