docs(internal): add technical docs for Stencil

[christian-bromann]

What is the current behavior?

Currently, Stencil lacks comprehensive technical documentation for contributors who want to understand the internal architecture. While there is user-facing documentation, contributors have to dive into the source code without guidance to understand how different components work together.

GitHub Issue Number: N/A

What is the new behavior?

This PR adds comprehensive technical documentation for Stencil contributors in the /docs directory. The documentation covers:

• Overview (index.md): High-level architecture diagram and component descriptions
• CLI (cli.md): Command-line interface architecture and task system
• Compiler (compiler.md): Build pipeline, TypeScript transformation, bundling, and output targets
• Runtime (runtime.md): Component lifecycle, state management, virtual DOM, and render scheduling
• Dev Server (dev-server.md): Multi-process architecture, HMR protocol, and middleware system
• Hydrate (hydrate.md): Server-side rendering and static site generation
• Mock Doc (mock-doc.md): Lightweight DOM implementation for SSR
• Declarations (declarations.md): TypeScript type definitions with references to actual source files
• Scripts Build System (scripts.md): Build system for developing Stencil itself (using esbuild)
• Testing (testing-deprecated.md): Deprecated test runner with migration guides
• Screenshot (screenshot-deprecated.md): Deprecated screenshot testing with migration paths

The documentation includes Mermaid diagrams, code examples, and detailed explanations of internal architecture to help new contributors understand and navigate the codebase more effectively.

Documentation

This PR is the documentation itself. It also updates the main readme.md to better describe the technical documentation for contributors.

Does this introduce a breaking change?

• Yes
• No

Testing

• Verified all documentation files render correctly with proper Markdown formatting
• Tested all Mermaid diagrams render properly
• Confirmed all file path references are accurate
• Validated that the documentation structure aligns with the actual codebase structure
• Cross-referenced type locations in declarations.md with actual source files

Other information

This documentation is specifically targeted at contributors who want to understand Stencil's internal architecture. It's not user-facing documentation but rather technical documentation to help people contribute to the Stencil project itself.