docs: create a diagram to show how the components connect

[cardoe]

This diagram shows how the components connect and authenticate to each
other.

graph TB
    IDP["External Identity Provider<br/>(e.g. Azure Entra / LDAP)"]

    subgraph global["Global Cluster"]
        Dex["Dex<br/>(OIDC Broker)"]
        Nautobot["Nautobot<br/>(Network Source of Truth)"]
        Keystone["Keystone<br/>(Identity Service)"]
    end

    subgraph site["Site Cluster(s)"]
        Ironic["Ironic<br/>(Bare Metal)"]
        Placement["Placement"]
        Neutron["Neutron<br/>(Networking)"]
        Glance["Glance<br/>(Images)"]
        Nova["Nova<br/>(Compute)"]
        Cinder["Cinder<br/>(Block Storage)"]
    end

    IDP -->|"authenticates users"| Dex
    Dex -->|"OIDC"| Nautobot
    Dex -->|"OIDC"| Keystone

    Keystone -->|"auth token validation"| Ironic
    Keystone -->|"auth token validation"| Placement
    Keystone -->|"auth token validation"| Neutron
    Keystone -->|"auth token validation"| Glance
    Keystone -->|"auth token validation"| Nova
    Keystone -->|"auth token validation"| Cinder

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