Add cop to enforce developers adding field descriptions on GraphQL schema

Use rubocop to remind adding description to each GraphQL field.

GraphQL documentation is auto generated based on GitLab schema. To make it
better field descriptions need to be added so our renderer can fetch them and add to the documentation.

More context

The following discussion from !29998 (merged) should be addressed:

• @eread started a discussion: (+4 comments)

  @felipe_artur here is a built version:

  

  It looks like we just need a bunch of text for introductory paragraphs and descriptions now.

  I think the basics here are sound, and you should move this on to a maintainer.

  How do we get that extra text together?