gh-47798: Reorganize subprocess docs around run and run_pipeline subsections

Give run() and run_pipeline() their own headings so they appear in
the page sidebar, move the run_pipeline block after the Popen
constructor section, mention run_pipeline in the intro paragraph and
Exceptions section, and split DEVNULL/PIPE/STDOUT and the base
exception classes into a "Constants and base exceptions" subsection
so they are not nested under the run heading in the TOC.

Author: gpshead