Create a User Story for the Population Density Data API (Stable readiness prerequisite)

[albertoramosmonagas]

Problem description

The Population Density Data API is missing a template-compliant User Story artifact, which is a prerequisite to move the API from Initial (0.x) to Stable (1.x) readiness. This creates a documentation gap for maintainers, reviewers, and adopters, since the API lacks a canonical narrative describing the end-to-end usage, actors, constraints, and expected outcomes.

Expected action

Create and add a User Story document to the repository using the agreed template. The User Story MUST include the following mandatory fields (as table rows):

• Summary
• Roles, Actors and Scope
• Pre-conditions
• Activities/Steps
• Post-conditions
• Exceptions

Additional context

• Use the agreed User Story table format already used in other CAMARA repositories.
• This issue is specifically to unblock Stable readiness documentation requirements (not to change API behavior).

CC: @camaraproject/population-density-data_codeowners