Document the approach to limiting query depth

[zerolab]

Refs:

• Offer a way to calculate and limit the total cost of a query graphql-python/graphene#772
• https://github.com/saleor/saleor/blob/fa288358ab8dfecb0f862c12edbc512ad0f3f000/saleor/graphql/views.py#L296-L302

#325 could help with some of the nesting for pages

[kbayliss]

Graphene now includes optional, configurable validation for this, so I don't think we need to do anything within grapple.

To limit query depth, you can append the rule to a custom GraphQLView, e.g.:

from typing import List

import graphql
from django.conf import settings
from graphene import validation as graphene_validation
from graphene_django import views as graphene_django_views
from graphql.error import graphql_error
from graphql.language import ast


class CustomGraphQLView(graphene_django_views.GraphQLView):
    ...

    def validate_query(self, document_ast: ast.DocumentNode) -> List[graphql_error.GraphQLError]:
        return graphql.validate(
            schema=self.schema.graphql_schema,
            document_ast=document_ast,
            rules=[
                graphene_validation.depth_limit_validator(
                    max_depth=settings.MAX_GRAPHQL_QUERY_DEPTH
                ),
            ],
        )

[zerolab]

@kbayliss that's neat. Updated the issue to reflect that this is now a documentation issue.
IMHO we want to document this as not everyone will scour the graphene docs