docs: add first guide for schema governance series

[sarahxsanders]

Description

adds the first guide for the new "schema governance" guides for graphql.org/learn

includes new guide: review and validate schema changes:

• defining review criteria
• automate validation
• CI/CD integration
• ownership and approval process
• breaking changes
• scaling
• pitfalls

Note

PLEASE DO NOT MERGE, THIS IS THE FIRST IN A THREE GUIDE SERIES

next guides are on:

• naming conventions and design standards: define consistent naming and documentation patterns
• schema ownership and governance: expand on the basics from this guide
• evolve schemas with versioning and tooling

[vercel]

@sarahxsanders is attempting to deploy a commit to the The GraphQL Foundation Team on Vercel.

A member of the Team first needs to authorize it.