docs: add device flow documentation

[nsklikas]

Related Issue or Design Document

Adds docs for the device flow as implemented in ory/hydra#3912.

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• 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 approval (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 the necessary documentation within the code base (if appropriate).

Further comments

I am not sure if this is the right place to place the doc, The rest of the docs in that folder do not reference hydra configurations, but the docs in the hydra folder do not seem to document oauth2/oidc flows. Please let me know if there is a better place for it.