Add “Apollo GraphQL server” to examples

[adamschwartz]

https://developers.cloudflare.com/workers/templates/pages/graphql_server/

Clean up can be done either in place in the existing Template Gallery, or in a PR in this repo as a new piece of content (can put it anywhere temporarily).

[kristianfreeman]

this is a weird one because any of these pages that have longer-form content live in the current docs repo – https://github.com/cloudflare/workers-docs/blob/master/workers-docs/src/content/templates/pages/graphql_server.md – it would probably make more sense if it lived in the template gallery, but grabbing markdown from the template registry as part of the gatsby build process sounds like big scope creep. ok to have this ticket here for now i think.