feat(core): cite source documents in the AI answer (#9863)

Support issue [BS-2424](https://linear.app/affine-design/issue/BS-2424).

### What changed?
- Add relevant document prompt templates.
- Add citation rules in system prompts.
- Change message `params` type to `Record<string, any>`
- Add unit test.

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packages/backend/server/src/plugins/copilot/providers/perplexity.ts

@@ -120,7 +120,7 @@ export class CitationParser {
   }
 
   public end() {
-    return this.flush() + this.getFootnotes();
+    return this.flush() + '\n' + this.getFootnotes();
   }
 
   private flush() {
@@ -135,7 +135,7 @@ export class CitationParser {
         citation
       )}"}`;
     });
-    return '\n\n' + footnotes.join('\n\n');
+    return footnotes.join('\n');
   }
 
   private getTokenContent() {