Add GNUInstallDirs and BINDIR/DOCDIR updates

[jopadan]

• Add include(GNUInstallDirs) and use ${CMAKE_INSTALL_BINDIR} or ${CMAKE_INSTALL_DOCDIR}
• Update .gitignores with source tree build garbage allowing git add -A, git commit -a and cmake . --install-prefix=/usr
• Skip TBB and embree install by default

TODO

• #449

[ericwa]

So to summarize, the goal here is to make make install usable on Linux etc. systems, and it's currently not?

This sounds good, thanks. It's going to break our current GitHub Actions packaging though.

• We'll need a cmake option() for using ${CMAKE_INSTALL_BINDIR} / ${CMAKE_INSTALL_DOCDIR} instead of . as it was previously. Maybe something like EWT_USE_GNU_INSTALL_DIRS. It can default to YES but the CI scripts (build-*.sh/ps1) will need to pass NO.
• The CI scripts will need to pass -DSKIP_TBB_INSTALL=NO -DSKIP_EMBREE_INSTALL=NO.
• Deleting the sphinx makefile is fine
• The .gitignore change I'm not a fan of... in-source builds are discouraged / not idiomatic CMake, so this shouldn't be necessary IMO. Can you just do an out of source build instead e.g. mkdir build && cd build && cmake ..?