docs: explain the framework's architecture

Add docs/dev/Architecture.md covering:
- Repository layout and crate dependency graph
- Responsibilities of all 11 crates
- Full build pipeline (SSR + SSG + client hydration)
- SSR request lifecycle (rendered once at startup via MetaCall)
- Dev mode hot-reload flow (two-channel broadcast architecture)
- Polyglot API routes (src/api/*.js via MetaCall)
- MetaCall integration points
- Key data structures (Manifest, SourceDirContainer, PageEntry, RebuildType)

Also update docs/README.md to link to the new developer docs section.

Closes #19

Author: Sukuna0007Abhi

docs/README.md

@@ -5,6 +5,7 @@ Welcome to the official documentation for MetaSSR Framework. This guide will hel
 ## Table of Contents
 - [Table of Contents](#table-of-contents)
 - [Getting Started](#getting-started)
+- [Developer Documentation](#developer-documentation)
 - [Contributing](#contributing)
 
 ---
@@ -26,6 +27,14 @@ We welcome contributions from the community! Visit the [Contributing](../CONTRIB
 - [Contributing Guide](../CONTRIBUTING.md)
 - [Code of Conduct](../CODE_OF_CONDUCT.md)
 
+---
+
+## Developer Documentation
+
+Internal documentation for contributors who want to understand MetaSSR's internals.
+
+- [Architecture](./dev/Architecture.md) — High-level design, crate responsibilities, build pipeline, SSR request lifecycle, hot-reload flow, and MetaCall integration points.
+
 
 ---