Draft: Split GraphQL docs into sub-pages

What does this MR do and why?

This MR splits GitLab's GraphQL documentation into separate pages, by type of GraphQL object.

Split GraphQL API reference documentation into ... (#593121)

Note

This MR will be split up into chunks when ready to merge.

• Index: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental
• Queries: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/queries
• Mutations: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/mutations
• Objects: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/objects
• Input objects: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/input_objects
• Scalars: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/scalars
• Enums: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/enums
• Interfaces: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/interfaces
• Unions: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/unions
• Directives: https://docs.gitlab.com/upstream-review-mr-ee-130378/api/graphql/reference/experimental/directives

Screenshots or screen recordings

Type	Before	After
Queries
Mutations
Objects
Input Objects
Scalars
Enums
Interfaces
Unions

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.