Add embedded signing support with webhook configuration

- Add helper function to generate embedded signing URLs from Documenso links
- Store embedded signature links for all signers (Client, Developer, CC)
- Add webhook secret configuration for embedded signing
- Add documentation for embedded signing and website implementation
- Update Interest type with embedded signature link fields

utils/types.ts

@@ -127,6 +127,10 @@ export interface Interest {
   "Signature Link Client"?: string;
   "Signature Link CC"?: string;
   "Signature Link Developer"?: string;
+  // Embedded signature link fields
+  "EmbeddedSignatureLinkClient"?: string;
+  "EmbeddedSignatureLinkCC"?: string;
+  "EmbeddedSignatureLinkDeveloper"?: string;
   // Documenso document ID for generated documents
   "documensoID"?: string;
 }