docs(api-server): add explanation for 'usage' filter in country model

- Include an Aside component to explain the purpose and benefits of using the 'usage' filter for country data retrieval
- Emphasize how this filter enhances user experience and optimizes data display by providing relevant country options
- Illustrate examples of filtering 'event countries' in headlines and 'headquarters' of sources

Author: fulleni

src/content/docs/api-server/features/data-management-api.mdx

@@ -29,6 +29,12 @@ GET /api/v1/data?model=country&filter={"usage":"eventCountry"}
 ```
 This fetches countries that are referenced as 'event countries' in headlines. Similarly, `filter={"usage":"headquarters"}` can be used to fetch countries that are headquarters for sources.
 
+<Aside type="note" title="Why use the 'usage' filter for countries?">
+The `usage` filter for the `country` model is designed to enhance the client-side user experience and optimize data retrieval. Instead of fetching a comprehensive list of all countries globally, this filter allows client applications to retrieve only those countries that are *relevant* to existing content.
+
+For instance, when a user wants to filter headlines by the country where the news event occurred, it's often more practical to present a list of countries for which actual headlines exist. Similarly, when filtering by the headquarters of news sources, displaying only countries that host active sources provides a more focused and actionable selection. This approach avoids presenting users with a long list of irrelevant options, streamlining navigation and improving the efficiency of data display.
+</Aside>
+
 ### The Dual Registry System
 
 The power behind this generic endpoint comes from two central registries: