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

src/pl/plperl/README

@@ -6,5 +6,4 @@ To build from this directory use 'make all; make install'.  libperl
 must have been built as a shared library, which is usually not the
 case in standard installations.
 
-Consult the PostgreSQL User's Guide and the INSTALL file in the
-top-level directory of the source distribution for more information.
+Consult the PostgreSQL User's Guide for more information.

src/tools/RELEASE_CHANGES

@@ -107,6 +107,9 @@ Starting a New Development Cycle
   placeholder), "git rm" the previous one, and update release.sgml and
   filelist.sgml to match.
 
+* In the newly-made branch, replace "devel" with the branch's major version
+  number in the URLs appearing in the top-level README and Makefile files.
+
 * In the newly-made branch, change src/backend/nodes/gen_node_support.pl
   to enforce ABI stability of the NodeTag list (see "ARM ABI STABILITY
   CHECK HERE" therein).