Update docker compose and readme

[Maghnie]

There are some instructions and settings that can be adapted to make following the tutorial easier for new users.

• Add OIDC as a Kong Plugin in the docker compose
• Update the keycloak container reference
• Include instructions for the latest Keycloak version in the readme

[Maghnie]

@SBlechmann

For the readme, what do you think about switching the order of the Keycloak and Kong/Konga sections?

Right now, the Kong/Konga instructions are described before the Keycloak instructions.
But configuring the OIDC plugin in Konga (section 3.3) references user secrets, which I was able to create only after following the Keycloak section (4).