[Automated][azure-resource-manager] Add Agent base type how-to guide

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Summary

Incremental documentation update for @azure-tools/typespec-azure-resource-manager. The library recently added the experimental base types feature (Azure.ResourceManager.BaseTypes), including the Agent base type with Agent, AgentConversation, and AgentResponse resource templates, Appliance/Platform deployment models, the @azureBaseType decorator, and two new lint rules (arm-agent-base-type-child-resources, arm-agent-base-type-lifecycle-operations).

The auto-generated reference docs (reference/decorators.md, data-types.md, linter.md) already reflect these additions. This PR fills the remaining manual-documentation gap.

Changes

• New how-to guide website/src/content/docs/docs/howtos/ARM/agent-base-type.mdx:
  • Explains base types and the @azureBaseType decorator / BaseTypeInfo / BaseType enum.
  • Documents the Agent, AgentConversation, and AgentResponse templates and the child-resource / lifecycle requirements enforced by the two new lint rules.
  • Contrasts the Appliance (service-owned, read-only) and Platform (client-owned, writable) deployment models and the HasModelDeploymentRef / HasInstructions definition parameters.
  • Shows how to suppress the basetypes-experimental warning.
  • References the canonical resource-manager/resource-types/agent sample via <ExampleRef>.
  • Auto-appears in the Howtos sidebar (autogenerated from the howtos directory).
• Knowledge base updated with notes on the base-types feature, the experimental suppression, and the fact that no per-rule website pages exist (link to reference/linter.md instead).
• Metadata (azure-resource-manager.meta.json) updated to record the checkout commit for the next incremental run.

Verification

• Built the ARM package and ran pnpm regen-docs — reference docs were already current (no diff).
• Every template, model, decorator, mix-in, and rule referenced in the guide was verified against lib/base-types/*.tsp, lib/private.decorators.tsp, the two rule .md files, and the resource-types/agent sample.
• cspell and prettier run clean on the changed files.

No publishable package source was modified (docs-only), so no changelog entry is required.

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