task: document OpenFGA integration on platform-mesh.io

[mirzakopic]

Description

Create documentation for the OpenFGA integration component as part of the Platform-Mesh public documentation at https://platform-mesh.io.

The content belongs under Overview > Components > OpenFGA integration as a dedicated subsection.

Suggested content

• Purpose and responsibility within Platform-Mesh
• Key concepts, CRDs, or resources it manages
• How it integrates with other Platform-Mesh components
• Installation / configuration notes (Helm values, flags, env)
• Links to the component repo, related RFCs, and API references

References

• Component repo: external (OpenFGA); config in platform-mesh/helm-charts
• Parent epic: epic: Stabilization and Preparation for Platform-Mesh 0.3 #201