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 https://gitlab.com/kicad/code/kicad/issues/7874

common/paths.cpp

@@ -240,6 +240,22 @@ wxString PATHS::GetUserCachePath()
 }
 
 
+wxString PATHS::GetDocumentationPath()
+{
+    wxString path;
+
+#if defined( __WXMAC__ )
+    path = GetOSXKicadDataDir();
+#elif defined( __WXMSW__ )
+    path = Pgm().GetExecutablePath() + "../share/doc/kicad";
+#else
+    path = wxString::FromUTF8Unchecked( KICAD_DOCS );
+#endif
+
+    return path;
+}
+
+
 bool PATHS::EnsurePathExists( const wxString& aPath )
 {
     wxFileName path( aPath );