Document alternatives to password grant for getting access token

[soxofaan]

(related to #191)

As far as I know https://documentation.dataspace.copernicus.eu/APIs/Token.html is the only documentation about how to get access tokens that can be used with CDSE. However, it only talks about the password grant, which is considered to be one of least favorable OIDC/OAuth grants because of poor security (it triggers non-savvy users to put their private credentials in scripts, source code and version control).

Ideally, the password grant should not be available in the first place. But at least, better alternatives (such as the device code grant) should be documented first (with the password grant being flagged as least favorable)

[soxofaan]

related ticket: https://jira.cloudferro.com/browse/CDSE-628