docs: add comprehensive getting started guide

- Add detailed infrastructure library development documentation
- Explain 183 module organization and 120+ exports structure
- Include workflows for adding utilities, constants, and env getters
- Document complex build system with external dependency bundling
- Add cross-platform compatibility requirements (Windows + POSIX)
- Explain 68 environment variable getters and 14 constant modules
- Include troubleshooting section for build and test issues
- Add vendoring workflow for external dependencies
- Provide quick reference tables and module categories
- Link to existing docs (build.md, CLAUDE.md)

This reduces new contributor onboarding time and provides a single
source of truth for infrastructure library development.

Author: jdalton