feat: Beginner Friendly GraphQL Doc

[Mitch-Henson]

This PR attempts to add some more backend / beginner friendly documentation on how to add a GraphQL endpoint for an existing Gravity endpoint. This was previously added in a previous PR but is being moved to its own PR to allow for more discussion without blocking any feature work.

As a first time user of GraphQL I definitely struggled even with the most basic of operations and hopefully this will help other future devs with their first PR.

[artsy-peril[bot]]

Warnings

:warning:

The V2 schema in this PR has breaking changes with Force. Remember to update the Force schema if necessary. Type 'UpdatePartnerArtworksMutationInput' was removed Type 'UpdatePartnerArtworksMutationPayload' was removed Type 'UpdatePartnerArtworksMutationType' was removed Type 'UpdatePartnerArtworksMutationFailure' was removed Type 'UpdatePartnerArtworksMutationSuccess' was removed Type 'UpdatePartnerArtworksMutationSuccessDetails' was removed Type 'PartnerArtworksBulkEditErrors' was removed Field 'matchArtist' was removed from object type 'Query' Field 'matchArtist' was removed from object type 'Viewer' Field 'updatePartnerArtworks' was removed from object type 'Mutation' Input field 'shippingTotalCents' was removed from input object type 'CommerceSellerAcceptProvisionalOfferInput' Input field 'shippingTotalCents' was removed from input object type 'CommerceSellerCounterOfferInput'

Generated by :no_entry_sign: dangerJS against 2c392e84b920deb0f16d62a9f375c96c3d63ab8e

[Mitch-Henson]

Thanks for everyone's feedback! I've made considerable updates, though it is not completely finished.

I would love to take you up on your pairing offer @damassi, but as I'm in Berlin, I think it is almost impossible. I've taken your very thorough feedback to heart and have moved a lot of things around. Let me know if the current Adding a GET request section is more of what you had in mind!

The Adding a POST request and Writing a mutation sections are still both empty, and I will have to pair with someone to make sure that I don't spread misinformation in this doc!