For Sage packages, the spkg-configure.m4 script and the distros directory don't seem to be documented. Remedy this.

Part of #27330 Meta-ticket: spkg-configure: Try to use as many system packages as possible

Component: documentation

Author: John Palmieri

Branch/Commit: e9e8a23

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30543