docs: escape {id} in api-server.md headings to fix MDX build (#1787)

MDX v2+ interprets curly braces in regular markdown as JSX
expressions. The headings 'GET /v1/responses/{id}' and
'DELETE /v1/responses/{id}' caused a ReferenceError during
Docusaurus static site generation because 'id' is not a
defined JavaScript variable. Escaped with backslashes.

Co-authored-by: Test <test@test.com>

website/docs/user-guide/features/api-server.md

@@ -136,11 +136,11 @@ Use the `conversation` parameter instead of tracking response IDs:
 
 The server automatically chains to the latest response in that conversation. Like the `/title` command for gateway sessions.
 
-### GET /v1/responses/{id}
+### GET /v1/responses/\{id\}
 
 Retrieve a previously stored response by ID.
 
-### DELETE /v1/responses/{id}
+### DELETE /v1/responses/\{id\}
 
 Delete a stored response.