Schema initialise task

[GuySartorelli]

There's too much boilerplate in setting up a default schema. Create a task to automate some of that process.

Acceptance criteria

• There's a tasks to make it easy to quickly initialise a GraphQL schema on the graphql dev tools module.
• This wonderful feature is highlighted in the doc.

PRs

• NEW Create GraphQLSchemaInitTask #53
• DOC GraphQL schema init task developer-docs#493
• NEW: Schema initialise task silverstripe-graphql#433
• DOCS: Document initialise graphql task silverstripe-framework#10218 (needs to be redone for developer-docs)

[sabina-talipova]

PRs have been merged. Task is completed.