Docs for Serializer/Deserializer missing

[dz902]

What did you find confusing? Please describe.

I could not find docs about how to use one in the docs. Also since setting content_type is not possible in v2, we will have to use Serializers but there are no docs.

Describe how documentation can be improved

Add them.

Additional context
Add any other context or screenshots about the documentation request here.

[dz902]

For those who encounter a situation where only content_type setting is needed, do the following:

from sagemaker.serializers import IdentitySerializer

...

    PREDICTOR = Predictor(
        endpoint_name = ENDPOINT_NAME,
        serializer = IdentitySerializer(content_type = 'application/x-image')
    )

[bveeramani]

Fixed by #1824.