Support content negotiation via `Accept` header

[francescobianco]

Description

The API already supports content negotiation through the Accept HTTP header, but this capability is currently undocumented.

When one of the following media types is specified, the default response format (application/json) is overridden and the output is returned accordingly:

Accept Header	Output Format
application/vnd.toon+text	Toon
text/markdown	Markdown
application/xml	XML
application/json	JSON (default)

Example

GET /api/resource
Accept: text/markdown

The response will be returned in Markdown format instead of JSON.

Proposed Action

• Document supported Accept header values.
• Add examples to the API documentation.
• Clarify that application/json remains the default response format when no supported Accept header is provided.