Use the installed location for COPYING.txt from sage.misc.copying.

Use src/doc/bootstrap to install a copy in src/doc somewhere as preparation for installation.

Perhaps autogenerate COPYING.txt from portions in build/pkgs/

Depends on #30912

CC: @williamstein @vbraun @jdemeyer @dimpase @tobihan @kiwifb @infinity0 @orlitzky

Component: build

Keywords: sd111

Issue created by migration from https://trac.sagemath.org/ticket/21571