Added `CONTROLLER_ENDPOINTS.md` to define REST API endpoints and
`REQUIRED_FEATURES.md` to outline required functions and features
for the `DocumentOperator` service. These documents include
detailed specifications for controllers, input/output formats,
usage scenarios, and technical requirements.
Updated `DocumentOperator.sln` to include the new documentation
files under a "Solution Items" section for better visibility.
Defined a prioritization strategy for feature implementation,
technical requirements, and a comprehensive test strategy.
Referenced relevant standards (e.g., PDF/A, ZUGFeRD) and
documented usage of the DevExpress Office File API.
