[EOEPCA/IAM] Update installation guide

[w-jfe]

The manual steps need to be added to the installation guide, i.e.

1. Create realm
2. Configure GitHub as an IdP
3. Create OPA client
   Details like realm name and client names should also be added.

The standard installation process should be based on the new overall Helm chart instead of the single component Helm charts. However, the latter should still be covered for completeness.

Furthermore, the content of the installation guide is currently repeated multiple times, which needs to be fixed.

[rconway]

Note that these aspects in the Deployment Guide (2.0-beta branch) may be relevant...

• Realm - https://eoepca.readthedocs.io/projects/deploy/en/2.0-beta/building-blocks/iam/#5-keycloak-post-deployment-configuration
• GitHub IdP - https://eoepca.readthedocs.io/projects/deploy/en/2.0-beta/building-blocks/iam/#b-add-github-as-a-keycloak-identity-provider
• OPA Client - https://eoepca.readthedocs.io/projects/deploy/en/2.0-beta/building-blocks/iam/#a-create-keycloak-client-for-opa