Clean deprecated extratool from configure and Makefile.in; update README with user-friendly walkthough

[kaiwu-astro]

1. It looks like the extratools has been deprecated, and hasn’t been maintained for very long. ./configure --enable-tools and make installtools will fail (cannot find file). It may now be proper that we clean the options from Makefile.in and configure script including in the help messages to avoid confusing.
2. Our documentation PDF will not be in doc/nbody6++_manual.pdf for long; it’s currently a .txt file there, so make install (some users love make install) will give an error. In Makefile, I change it to cp ./doc/nbody6++_manual.* $(INSTALLDIR)/share/doc/, so future PDF (if so) will also work
3. Our executable name may have different suffix like gpu, mpi, avx. In Makefile.in, I link the executable to nbody6++. Now after make install, the user can call nbody6++, no matter that the suffix is.
4. Add a new small input file (10k, 2Myr, no dat.10) for new users to quickly try a test simulation (examples/input_files/N10k_noDat10.inp)
5. Update README file: now potential new users can start with the detailed walkthough there.