Fix nested groups and global class output

[auscaster]

Summary

• Parse Doxygen nested groups and render group lists plus breadcrumbs in CLI and generated-page output.
• Make --groups and --classes compose so group files and standalone class files are both written.
• Preserve global-scope classes and root globals in split output via api.md, with docs and examples updated.
• Add a regression fixture covering top-level groups, nested groups, root globals, and global classes.

Tests

• npm run build
• npm test
• npm run example

Fixes #103.
Fixes #104.
Fixes #105.