Trac #25206: Update README.md and Installation Manual for 9.1, integr…

…ate instructions for Windows (WSL and Cygwin) in manuals

Followup to #22450 to improve any hard-coded (e.g. in the official Sage
docs, as opposed to on a the wiki) documentation concerning installing
Sage for Windows, in particular to emphasize the native binary.

Cygwin source material:
- https://trac.sagemath.org/wiki/Cygwin64Port

WSL source material:
- #28549 Make it possible to build on WSL

See also:
 - #29557: Add script package _recommended and generate "recommended
system packages" info

URL: https://trac.sagemath.org/25206
Reported by: embray
Ticket author(s): Matthias Koeppe
Reviewer(s): Michael Orlitzky