chore(docs): Add contact merge endpoint to swagger documentation (#13172)

## Summary

This PR adds API documentation for the contact merge endpoint:

`POST /api/v1/accounts/{account_id}/actions/contact_merge`

This endpoint allows merging two contacts into one. The base contact
survives and receives all data from the mergee contact, which is then
deleted.

## Changes

- Added `swagger/paths/application/contacts/merge.yml` with complete
endpoint documentation
- Updated `swagger/paths/index.yml` to include the new endpoint

## Related Issues

Closes chatwoot/docs#243

---------

Co-authored-by: Pranav <pranav@chatwoot.com>

swagger/paths/index.yml

@@ -275,6 +275,12 @@
 /api/v1/accounts/{account_id}/contacts/{id}/contactable_inboxes:
   $ref: ./application/contactable_inboxes/get.yml
 
+# Contact Merge
+/api/v1/accounts/{account_id}/actions/contact_merge:
+  parameters:
+    - $ref: '#/components/parameters/account_id'
+  post:
+    $ref: ./application/contacts/merge.yml
 
 # Automation Rule
 /api/v1/accounts/{account_id}/automation_rules: