Document the process for Marketplace as an OIDC Identity Provider

[flongford]

Since Marketplace is now using a KeyCloak client for user authentication and authorization, it will be beneficial for downstream apps to provide document for this service.

Potential scenarios are:

• How does an application vendor validate a Marketplace user's access token?
• How does an application vendor configure their application to use KeyCloak's authorization code flow to support Marketplace as an Identity Provider?

The well-known folder should contain all the OIDC endpoints required:

https://www.materials-marketplace.eu/auth/realms/marketplace/.well-known/openid-configuration