[mangadex] implement login with client credentials

unless the legacy method still works for your account,
login now requires 4 values:

- client-id
- client-secret
- username
- password

where 'client-id' and 'client-secret' are the credentials
of a personal API client.

> Public clients are not yet available.

docs/configuration.rst

@@ -7566,6 +7566,23 @@ How To
       as ``"api-key"`` and ``"api-secret"``
 
 
+extractor.mangadex.client-id & .client-secret
+---------------------------------------------
+Type
+    ``string``
+How To
+    * login and go to your `User Settings <https://mangadex.org/settings>`__
+    * open the "API Clients" section
+    * click "``+ Create``"
+    * choose a name
+    * click "``✔️ Create``"
+    * wait for approval / reload the page
+    * copy the value after "AUTOAPPROVED ACTIVE" in the form "personal-client-..."
+      and put it in your configuration file as ``"client-id"``
+    * click "``Get Secret``", then "``Copy Secret``",
+      and paste it into your configuration file as ``"client-secret"``
+
+
 extractor.reddit.client-id & .user-agent
 ----------------------------------------
 Type