Mark the Eclipse USS API Documentation as Deprecated

Via eclipsefdn/helpdesk#4696, we have agreed to deprecate the Eclipse User Storage Service (USS) by July 2, 2025.

One of our tasks is to implement a deprecation notice in our API documentation to inform current users. Our USS API documentation is currently available here: USS API Documentation.

To avoid any confusion, I propose we extract the USS specification and create a standalone file exclusively for USS, where we can add the following notice at the top:

Deprecation Notice

Important: The Eclipse User Storage Service (USS) API will be taken offline on July 2, 2025. After this date, the service will no longer be supported or maintained. Projects are encouraged to transition to alternative solutions as soon as possible. For more information, please refer to our GitLab Helpdesk Issue #4696.