Move IAM federation doc to an ADR

[garloff]

In PR #72, we merged docs on how federation is designed to be supported in SCS.
Reading through the texts, the impression came up that these are decision type documents that are Architecture Decision Records in the standards repository. So let's understand the intended nature of the document better (end user reference documentation vs ADR vs How-To) and then determine the final target destination.

[reqa]

Currently in https://github.com/SovereignCloudStack/docs/blob/main/dev-docs/operations/iam/openstack-federation-via-oidc.md , which to me looks like oriented towards SCS-developers. My intention with this was to explain to "technical SCS operators" (my wording) how things are intended to work in SCS, so they can e.g. adjust Keycloak config and Keycloak to Keystone mapping as needed. I'm also fine to move it into the "standards" section if the target audience matches. We can also document federation to SCS devs, but my intention was to provide a starting point for operators.

OTOH we have https://github.com/SovereignCloudStack/docs/blob/main/docs/05-iam/intra-SCS-federation-setup-description-for-osism-doc-operations.md , which is even more technical and will stay on that technical level for the foreseeable future. I guess moving that one into a "standard"/ADR would make more sense.

In a quick chat @fkr suggested to split it into two parts:

• technical "how are things done" points should go into an ADR
• operational "what do I need to do to achieve XYZ" should go into the operation docs