docs: explain comment handling during sorting

[Himanshuagrawal4]

Summary

• document how leading, same-module, multiline, and inline import comments are rendered
• link to action comments and the configuration options that affect comment handling
• add the new page to the documentation index

Closes #1843

Validation

• compared all four documented transformations with the current isort.code output
• built the complete Sphinx documentation environment with warnings treated as errors using the network-independent dummy builder