Moved installation notes to the wiki

[mahrud]

As discussed in #3294:

• deleted INSTALL.mac in favor of Building-M2-from-source-on-macOS
• deleted INSTALL in favor of Building-M2-from-source-using-Autotools
• deleted INSTALL-CMake.md in favor of Building-M2-from-source-using-CMake
• renamed README -> README.md
• deleted obsolete gawk script

The autotools install page on wiki still needs some work, but at least it contains all the information previously in INSTALL.

Skipping CI builds because only touching text files and an obsolete gawk file not referenced from anywhere (used to be used by GNUmakefile.in but it has long been removed).