Replace duplicated GC, compiler, and VM details with concise
summaries linking to gc.md, compiler.md, and vm.md.
Co-authored-by: Claude <noreply@anthropic.com>
Three new documents:
- doc/guides/getting-started.md: building, running, and embedding mruby
- doc/guides/capi.md: C API reference for values, classes, methods, etc.
- doc/internal/architecture.md: internal architecture for developers
Co-authored-by: Claude <noreply@anthropic.com>