update docs

- document '"mode": "jsonl"' for metadata pp
- remove 'username' and 'password' from 'instagram'
- add 'ytdl' example (#3180)

docs/gallery-dl.conf

@@ -159,9 +159,8 @@
         },
         "instagram":
         {
-            "username": null,
-            "password": null,
             "api": "auto",
+            "cookies": null,
             "include": "posts",
             "sleep-request": [6.0, 12.0],
             "videos": true