To add to the documentation for OAuth 2.0 Authentication

### Where is the problem?

https://docs.konghq.com/hub/kong-inc/oauth2/

### What happened?

I dont see documentation of various ways of calling "/oauth2/token" call for client_credentials flow.

### What did you expect to happen?

In Client credentials flow, there is a provision to make "oauth2/token" call by passing the client ID, client secret and grant_type as query param or a JSON body along with usual way of sending it in URL-encoded way.

### Code of Conduct and Community Expectations

- [X] I agree to follow this project's Code of Conduct
- [X] I agree to abide by the Community Expectations

Start by reviewing the OAuth 2.0 documentation at https://docs.konghq.com/hub/kong-inc/oauth2/ and confirm the client_credentials flow details for the /oauth2/token endpoint. Done means documenting the query-parameter, JSON-body, and URL-encoded request forms, while checking the replacement repository because this repository is archived.

Written by the indexing model from the issue text.