Add architecture diagrams for Radius services and CLI

[willdavsmith]

Description

Add architecture diagrams for Radius services and CLI

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Fixes: #issue_number

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document PR is created in the design-notes repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for the samples repository is created, if existing samples are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable
• A PR for the recipes repository is created, if existing recipes are affected by the changes in this PR.
  • Yes
  • Not applicable

[Copilot]

Pull request overview

Adds a code-oriented architecture documentation set under docs/architecture/ and links to it from existing contributor docs, so developers can quickly find entry points, key packages, and safe-change guidance when working in this repository.

Changes:

• Introduce new architecture “start here” index plus per-binary architecture notes (UCP, applications-rp, dynamic-rp, controller, CLI).
• Add shared-runtime/ARM-RPC overview and a service interaction map with Mermaid diagrams.
• Cross-link architecture docs from docs/README.md and the control-plane contributing index.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/README.md	Adds a pointer from docs index to the new architecture docs set.
docs/contributing/contributing-code/contributing-code-control-plane/README.md	Links control-plane contributor guide to architecture docs for upfront context.
docs/architecture/README.md	Provides the architecture docs entrypoint, reading order, and related material links.
docs/architecture/service-interaction-map.md	Adds a high-level runtime map of executables and typical flows.
docs/architecture/shared-runtime-and-armrpc.md	Documents shared hosting/HTTP/builder/async framework used by Go services.
docs/architecture/ucp.md	Adds UCP-focused entry points, package map, and routing flow diagrams.
docs/architecture/applications-rp.md	Adds applications-rp architecture overview and builder/async handoff flow.
docs/architecture/dynamic-rp.md	Adds dynamic-rp architecture overview emphasizing generic lifecycle handling.
docs/architecture/controller.md	Adds controller architecture overview and reconciler flow diagram.
docs/architecture/rad-cli.md	Adds CLI architecture overview, dependency seams, and deploy flow diagram.