docs: add functional modules and interactions document

[Copilot]

No documentation existed describing CM's internal structure or how its components relate to each other. This adds a reference document covering all functional modules and their interactions.

Changes

• doc/modules.md (new) — covers:

  • Both binaries (cluster-manager REST server, template-controller operator) with startup sequences
  • All 21 internal packages (rest, controller, auth, multitenancy, k8s, convert, providers, inventory, middleware, template, config, labels, pagination, metrics, logger, events, webhook, core, common, cluster, mocks) — each with path, role, and key responsibilities
  • Four ASCII interaction flow diagrams: REST request pipeline, ClusterTemplate reconciliation, project lifecycle (Nexus → namespace), and admission webhook validation
  • External integrations table (OIDC, Keycloak, OPA, Vault, infra-core inventory gRPC, Nexus gRPC, Kubernetes API)
  • Kubernetes custom resource catalogue: ClusterTemplate CRD schema, CAPI resources generated by the controller, and CAPI resources managed by the REST server

• doc/README.md — adds SPDX header and links to the new document

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