Documentation for #615 - PackageRevision readiness gates in Porch

[JamesMcDermott]

Documentation for #615

• new document describing operation of readiness gates on PackageRevisions
  • in general (no existing document for it)
  • specifically the changes introduced in Have PackageVariant set readiness gate on PackageRevisions porch#156
    • including default readiness gates managed by Porch server for all PackageRevisions and by package variant controller for PackageRevisions controlled by a PackageVariant
• new diagrams to illustrate flows

nephio-project/nephio#615
nephio-project/porch#156

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Quality Gate passed

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