Merge branch 'docs' into 'master'

docs/backend/api.md

docs/backend/models.md

docs/backend/models/ContainerUnion.md

docs/backend/models/Document.md

docs/backend/models/Group.md

docs/backend/models/Item.md

docs/backend/models/ItemDocument.md

docs/backend/models/Rental.md

docs/backend/models/Storage.md

docs/backend/models/User.md

docs/frontend/components.md

docs/readme.md