document motivation for EasyBuild configuration settings

[boegel]

We divert from the default EasyBuild configuration in various ways when installing software into EESSI.

We should add a documentation page that explains why we're doing this, per (group of) configuration settings that are non-default.

[ocaisa]

The full set of non-path ones (today) is

allow-loaded-modules                      (E) = EasyBuild, EESSI-extend
debug                                     (E) = True
experimental                              (E) = True
filter-deps                               (E) = Autoconf, Automake, Autotools, binutils, bzip2, DBus, flex, gettext, gperf, help2man, intltool, libreadline, libtool, M4, makeinfo, ncurses, ParMETIS, util-linux, XZ, zlib
filter-env-vars                           (E) = LD_LIBRARY_PATH
group-writable-installdir                 (E) = True
hooks                                     (E) = /cvmfs/software.eessi.io/versions/2023.06/init/easybuild/eb_hooks.py
ignore-osdeps                             (E) = True
read-only-installdir                      (E) = True
set-gid-bit                               (E) = True
sticky-bit                                (E) = True
sysroot                                   (E) = /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64
umask                                     (E) = 002
umask                                     (E) = 022
umask                                     (E) = 077
zip-logs                                  (E) = bzip2

This covers all the 4 scenarios, so there are some that are unique to specific scenarios.

These will change with EESSI/software-layer-scripts#45 so not worth documenting until that is resolved.