Add architectural overview document

[jtdub]

Description

The project has excellent deep-dive documentation for specific features (drivers.md, remediation-reporting.md), but there is no high-level architectural overview that explains how the core components fit together.

New users and contributors must piece together the architecture from multiple 300+ line documents.

Proposed Content

An architecture.md (or section in the docs site) covering:

1. Core tree model — How HConfig (root), HConfigChild (nodes), and HConfigChildren (collection) form the configuration tree
2. Driver system — How HConfigDriverBase and HConfigDriverRules encode platform-specific behavior, and how drivers plug into the tree
3. Workflow layer — How WorkflowRemediation uses config_to_get_to() to compute remediation and rollback configs
4. View layer — How HConfigViewBase provides a higher-level interface abstraction on top of the tree
5. Reporting layer — How RemediationReporter aggregates across multiple devices
6. Data flow diagram — running_config text → get_hconfig() → tree → WorkflowRemediation → remediation/rollback

This would serve as a map that helps users navigate the detailed docs and helps contributors understand where new code should go.