Add description for Developers/webhook page (#6327)

- Add optional description field to webhook page in developer settings.

Fix https://github.com/twentyhq/twenty/issues/6236

---------

Co-authored-by: Thomas Trompette <thomas.trompette@sfr.fr>

packages/twenty-server/src/engine/core-modules/open-api/utils/computeWebhooks.utils.ts

@@ -22,6 +22,10 @@ export const computeWebhooks = (
                   type: 'string',
                   example: 'https://example.com/incomingWebhook',
                 },
+                description: {
+                  type: 'string',
+                  example: 'A sample description',
+                },
                 eventType: {
                   type: 'string',
                   enum: [