Sometimes someone may build only a subset of the documentation. The
logic has to be conditional upon whether it exists, not whether it is
built, since we pull pre-built docs in for CI installation.
This is a massive rewrite of the help text and cli.rst section of the
manual. All command-line flags now have their own help and are
specifically index. qpdf --help is completely redone.
Also remove linearization from qpdf-manual.pdf. It's a small file, and
removing the dependency on the qpdf executable significantly shortens
build times.
