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 <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Signed-off-by: Rob Herring <[email protected]>
Reviewed-by: Mauro Carvalho Chehab <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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