Add a way to configure the documentation search path

[Peter TB Brett]

Currently, gschemdoc (the Scheme program used by gschem to look up per-component documentation) searches three directories on the local machine:

1) The current working directory. This is usually the directory where the schematics live.

2) A system documentation directory, usually $PREFIX/share/doc/geda-gaf. This isn't super useful, because regular users can't actually put anything there.

3) A user documentation directory, $XDG_DATA_HOME/doc/geda-gaf (i.e. almost always $HOME/.local/share/doc/geda-gaf). This is also not super useful, because usually you want to keep datasheets somewhere near the symbols.

An "easy win" would be to add system, user and project configuration variables that control the documentation search path. The default values could match the current settings.