Add blog post on external IdP configuration

Several blog posts are already published on the docs website: https://docs.nmaas.eu/blog/

We should add another one focused on how to connected an nmaas instance to an OIDC compliant external IdP.

Few point to consider/include:

• brief intro to "external IdPs", SSO, OIDC, maybe Keycloak (mostly references to external sources)
• how to configure an IdP for an nmaas installation (what properties need to be set, take a look at the Helm chart repo)
• write that Keycloak is used for our central nmaas instances
• share the observed issues with different IdP supporting different parameters