make table of contents 4 levels deep

Daniel Kahn Gillmorrequested to merge
toc-depth-4 into main

This addresses the minor side-note from Derek Atkins that the table of contents of RFC 4880 contains 4 levels of depth: https://mailarchive.ietf.org/arch/msg/openpgp/1b_fJlpHpioO1YOJnCyx2kayyIM/

This is a minor structural edit, not a substantive change.

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