User Guide update

I went through all of the schema user guide and much of the mr user guide and fixed the egregious issues (i.e. commands, code which just won't work anymore or never worked). There might still be issues, but nothing to prevent release IMHO.

@jhlch thanks again for agreeing to review

@kryzthov @ajprax @kimballa Anything I missed?

Note: consider the latest commit: Schema User Guide contains ls, get, scan.
Everything else is older.