Merge branch 'docs' into 'master'

.gitlab-ci.yml

README.md

docs/.gitignore

docs/backend/api.md

docs/backend/api/GroupResolver.md

docs/backend/api/ItemDocumentResolver.md

docs/backend/api/ItemResolver.md

docs/backend/api/MemberResolver.md

docs/backend/api/RentalResolver.md

docs/backend/api/StorageResolver.md

docs/backend/api/UserResolver.md

docs/backend/api/template.md

docs/backend/auth.md

docs/backend/models.md

docs/backend/models/ContainerKind.md

docs/backend/models/ContainerUnion.md

docs/backend/models/Document.md

docs/backend/models/Item.md

docs/backend/models/ItemDocument.md

docs/backend/models/Rental.md