docs(api-server): update Source model to use embedded Language object

- Replace `language` field of type string with an embedded Language object
- Add new Language model with `code`, `name`, and `nativeName` fields
- Update existing models and sections to reflect the new Language object

Author: fulleni

src/content/docs/api-server/reference/core-models.mdx

@@ -55,7 +55,7 @@ Represents a news publisher.
 -   `description` (string): A brief description of the source.
 -   `url` (string): The homepage URL of the source.
 -   `sourceType` (string): The type of source (e.g., `newsAgency`, `blog`).
--   `language` (string): The primary language of the source (e.g., "en").
+-   `language` (Language): The embedded language object.
 -   `headquarters` (Country): The embedded country object where the source is based.
 
 ### Country
@@ -65,3 +65,11 @@ Represents a country.
 -   `isoCode` (string): The two-letter ISO 3166-1 alpha-2 code (e.g., "US").
 -   `name` (string): The common name of the country (e.g., "United States").
 -   `flagUrl` (string): A URL to an image of the country's flag.
+
+### Language
+
+Represents a language.
+
+-   `code` (string): The ISO 639-1 code for the language (e.g., "en", "es").
+-   `name` (string): The common name of the language (e.g., "English", "Spanish").
+-   `nativeName` (string): The name of the language in its own tongue.