Added documentation for the keycloak-oidc provider and the new Keycloak admin console #1931 (#1999)

* Added documentation for the keycloak-oidc provider in regard to the new Keycloak admin console "Admin2". As of v19.0.0 it is the default web console and OAuth2 proxy documentation has been updated to show end-users how to create a sample test Keycloak OIDC client to integrate with Oauth2 Proxy.

* Issue #1931
Added documentation for the keycloak-oidc provider in regard to the new Keycloak admin console "Admin2". As of v19.0.0 it is the default web console and OAuth2 proxy documentation has been updated to show end-users how to create a sample test Keycloak OIDC client to integrate with Oauth2 Proxy.
Added a link in the documentation to older keycloak-oidc pull request, as the provider currently evaluates aud from the access token and not the id token.

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Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>