graphql: add mutations howto
graphql: add mutations howto

Change-Id: I5449378c43ac48c52d9219f1c45c8e78a0bffc80
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/221158
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Landon Gilbert-Bland <60d9ff23682c863d36b48b4c32c5ce5f434d29c0@instructure.com>
QA-Review: Landon Gilbert-Bland <60d9ff23682c863d36b48b4c32c5ce5f434d29c0@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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