Document '/attribute' API routes with OpenAPI

[eduardoj]

Introduce the new "Attributes" group of operations. This group contains the documentation of all operations with API routes starting by '/attribute'.

To verify the documetation that has been introduced:

• Start the development environment
• Go to http://localhost:3000/apidocs-new/

---

If this PR requires any particular action or consideration before deployment,
please check the reasons or add your own to the list:

• Requires a database migration that can cause downtime. Postpone deployment until maintenance window[1].
• Contains a data migration that can cause temporary inconsistency, so should be run at a specific point of time.
• Changes some configuration files (e.g. options.yml), so the changes have to be applied manually in the reference server.
• A new Feature Toggle[2] should be enabled in the reference server.
• Proper documentation or announcement has to be published upfront since the introduced changes can confuse the users.

[1] https://github.com/openSUSE/open-build-service/wiki/Deployment-of-build.opensuse.org#when-there-are-migrations
[2] https://github.com/openSUSE/open-build-service/wiki/Feature-Toggles-%28Flipper%29#you-want-real-people-to-test-your-feature

[obs-bot]

Review app will appear here: http://obs-reviewlab.opensuse.org/eduardoj-document_attribute_routes_with_openapi

[hennevogel]

In general, can we order the documentation by?

• List
• Show
• Create/Update
• Delete