intro.1: 2024 edition

Modernize intro.1, attempting to preserve style and brevity,
including a paragraph about installing more commands, a FILES
section explaining where the commands are located and why, and
adding section number to HISTORY for clarity.

Advances: #248562
Reviewed by: imp, mhrone
Pull Request: https://github.com/freebsd/freebsd-src/pull/1136