docs:add client credentials grant how-to

[narg95]

Related issue

Issue #566

Proposed changes

This PR adds an initial How To to configure Fosite as an OAuth2 Provider to issue RSA-JWT access tokens using Client Credentials Grant

This is the first attempt to improve fosite's documentations with practical how-tos to customize it. Your input at this phase is very welcome :)

The idea would be to add multiple how-to files into the docs/how-tos/ folder, and once they are in a good shape, we link them in the README.md.

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

[aeneasr]

Awesome, thank you!