Add Tree View for Scaffolded Website Structure

## Summary

As discussed on https://github.com/finos/architecture-as-code/issues/2039 as part of the demo of the create website mode [pull/2061](https://github.com/finos/architecture-as-code/pulls/2061), we should add a dedicated tree view in the VSCode extension sidebar that displays the scaffolded website structure. This allows users to navigate and manage their documentation site without relying on the standard file explorer.

## Problem Statement

After running "Create Website" (scaffold mode), users must use the file explorer to navigate the generated structure. This has limitations:

- The file explorer shows all files, not just the documentation structure
- No visual distinction between content pages, config files, and static assets
- Cannot see the logical hierarchy (nodes, relationships) at a glance
- No quick actions specific to documentation pages (edit, preview, delete from site)

## Current Experience

After scaffolding, users see this in the file explorer which they use to navigate.

```
website/
├── docs/
│ ├── index.md
│ ├── nodes/
│ │ ├── ecommerce-system.md
│ │ ├── payment-service.md
│ │ └── inventory-service.md
│ └── relationships/
│ └── sys-composition.md
├── static/
├── package.json
├── sidebars.js
└── docusaurus.config.js
```

## Proposed Experience

A new "CALM Website" tree view in the CALM sidebar: