Add architecture overview (Mermaid diagram) and link from README #390
Description
Problem
There is no visual, repo-local architecture overview that explains how Canton participant nodes interact via synchronization domains and how cross-domain workflow composition works.
Proposal
- Add docs/architecture/overview.md with a Mermaid diagram and a short description.
- Link this page from README.md (Architecture section).
Scope
- Docs-only change. No code or build files touched.
Rationale
- Easier onboarding for new contributors/integrators.
- Keeps a canonical high-level picture in-repo (alongside external docs).