Respect KICAD_DOCS when searching for help files

On Linux, the documentation and help files are potentially installed to
a non-standard location (i.e., outside of /usr/share/doc/kicad/). This
can be the case when, e.g., multiple versions of KiCad are installed in
parallel. Making KICAD_DOCS available at run-time is the only viable
solution to allow the applications to find the help files in this case.

Fixes #7874