Stop generating plain-text INSTALL instructions.

Stop generating plain-text INSTALL instructions.

Up to now, our distribution tarballs have included a plain-text form
of the installation.sgml chapter.  The rationale for that was that a
recipient might not have either ready internet access or HTML-viewing
tools; a theory that seems downright quaint today.  Maintaining the
ability to generate this file is not without cost, because it puts
special requirements on installation.sgml that are often overlooked.
Moreover, we are moving in the direction of making our distribution
tarballs be pure git snapshots for traceability/reproducibility
reasons; including generated files doesn't fit into that plan.
Hence, let's just drop INSTALL and remove the infrastructure for
generating it.  The top-level README will now recommend visiting
our website to see the installation instructions.  As a useful
side-effect, we can get rid of README.git which has provoked
confusion.

Discussion: https://postgr.es/m/20231220114927.faccqqprmuyrzdip@alap3.anarazel.de
Discussion: https://postgr.es/m/e07408d9-e5f2-d9fd-5672-f53354e9305e@eisentraut.org