Move GraphQL docs to be added as optional module in CMS 6

[emteknetnz]

When we have CMS 6 docs, include the silverstripe/graphql module as an optional module, similar to linkfield

Acceptance Criteria

• silverstripe/silverstripe-graphql is added to https://github.com/silverstripe/doc.silverstripe.org/blob/master/sources-docs.js and displays in the "Optional Features" section

PRs

• DOC Set metadata of docs base silverstripe-graphql#604
• ENH Add graphql as an optional module #310

[GuySartorelli]

PRs merged