Compute extensibility documentation overhaul and migration guide

[Copilot]

Initial documentation set for the new Radius compute extensibility architecture and a migration path off the legacy Applications.* / recipeConfig model. Legacy pages are retained and cross-linked since Radius.* APIs are still preview (2025-08-01-preview).

Description

• Concepts
  • concepts/compute-extensibility/ — new architecture, Radius.Compute/* Resource Types, decomposed Environment, control plane diagram
  • concepts/recipe-packs/ — Recipe Pack model, composition, override semantics
• How-to guides (guides/recipes/)
  • recipe-packs/ — author, deploy, reference, and compose Recipe Packs
  • howto-bicep-settings/ — Radius.Core/bicepSettings for private OCI registries (BasicAuth, AzureWI, AwsIrsa); replaces recipeConfig.bicep.authentication
  • howto-terraform-settings/ — rad terraform install, terraformrc, Tier 1/2 backends, env vars, custom provider secrets via Recipe parameters; replaces recipeConfig.terraform.*
• Migration (guides/operations/migration/)
  • _index.md — what-changed table, recommended ordering, full before/after Bicep for an Environment (legacy recipeConfig → recipePacks + bicepSettings + terraformSettings) and an Application (Applications.Core/containers → Radius.Compute/containers, etc.), validation commands
  • breaking-changes.md — enumerated breaking changes with mitigations: API namespace/version, Environment decomposition, hard-coded compute removal, Terraform binary lifecycle, custom provider model, backend support tiers, Bicep auth schema, secret schema, deprecation guidance
• Legacy cross-link — warning callout on howto-private-bicep-registry pointing to the new BicepSettings guide and migration guide

Example of the central decomposition documented in the migration guide:

// After
resource env 'Radius.Core/environments@2025-08-01-preview' = {
  name: 'my-env'
  properties: {
    recipePacks:       [myRecipePack.id]
    bicepSettings:     myBicepSettings.id
    terraformSettings: myTerraformSettings.id
  }
}

Where features remain in flight (e.g. radius-project/radius#11780), pages note preview status and link to the upstream design notes / feature spec for SMEs to finalize. Legacy Applications.* pages should be removed/redirected in a follow-up once the new APIs are promoted to stable.

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