[Improvement]: Update docs

[mcop1]

Improvement description

Update datahub / graphql documentation

• Update screenshots

  • replace screenshots with old graphiQL ui
    e.g. add mutation
  • replace screenshots with old pimcore ui
    e.g. configuration

• Check all sections marked with "ToDo"

  • 04 Asset Mutations
  • 22 Sample Advanced ManyToMany Object Relation
  • 23 Sample Advanced ManyTomany Relation Metadata
  • 20 DataObject Mutations

• Make existing content more detailed

  • add operators, transformers, others that are missing (e.g. merge)
  • Update security settings page
  • Add information about workspaces (no workspace selected -> no access)

[KatrinAckermannEk]

Most of the code snippets in the documentation are dark grey text on black background which makes them unreadable. (Only by selecting them)

[dvesh3]

@KatrinAckermannEk we are reviewing a PR to fix those issues pimcore/pimcore-docs#19 😉