Write a guide how heimdall can be used with Keycloak

[dadrus]

Preflight checklist

• [X] I agree to follow this project's Code of Conduct.
• [X] I have read and am following this repository's Contribution Guidelines."
• [X] I have discussed this feature request with the community.

Describe the background of your feature request

Keycloak is a very famous open source Identity and Access Management platform, which supports OIDC and SAML.

Even heimdall implements only the resource server functionality, it can easily be used in setups with Keycloak by bringing in an additional components like lua-resty-openidc, oauth2-proxy or similar

Describe your idea

Since many users are asking for a "turn-key" solution, write a guide showing possible setups to make decision taking easier.