Allow and make effective -j option in "make web"

- forbid simultaneous lilypond-book running instances in
make/ly-rules.make by adding a dependency chain of .texi output files;

- add $(src-dir) to Texi2HTML include path, so it finds macros.itexi;

- replace documentation PO phony target with a dummy target (to prevent
it from being excessively rebuilt), and make it a prerequisite for
tely->texi rules instead of tely source files (to make sure it is
actually built);

- remove superfluous $(outdir)/version.itexi prerequisites in WWW-2;

- update documentation on documentation building.