Building PDF files in parallel with a2x often fails

[clepple]

Ran into this while testing pull #418. A make -j10 with PDF output enabled will often fail due to parallel a2x processes using the same names for intermediate files.

Proposed solution: uniquely named temporary directories.

[StefanBruens]

In kicad this was "solved" by using a dependency between the various output formats. Thus different manuals are built in parallel, but e.g. user-manual.{txt,pdf} were built sequentially.

[bigon]

Adding .NOTPARALLEL: at the end of the Makefile in the docs/ directory workaround the problem

It will prevent that makefile to be run in parallel and prevent the build to fail

[jimklimov]

Should be fixed with #1154 (and possibly other contemporary efforts)