Add SKEL_DOC to the environment setup

The doc folder is now addressed via an environment variable like every other
project folder, so tools can resolve doc paths without hardcoding them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

bin/setenv.bat

@@ -20,6 +20,7 @@ REM Pfade fuer verschiedene Komponenten
 set "SKEL_BIN=%SPS_SKEL%\bin"
 set "SKEL_LIB=%SPS_SKEL%\lib"
 set "SKEL_DATA=%SPS_SKEL%\data"
+set "SKEL_DOC=%SPS_SKEL%\doc"
 set "SKEL_CFG=%SPS_SKEL%\cfg"
 set "SKEL_LOG=%SPS_SKEL%\log"
 set "SKEL_TESTS=%SPS_SKEL%\tests"
@@ -52,6 +53,7 @@ echo SKEL_BIN          = %SKEL_BIN%
 echo SKEL_CFG          = %SKEL_CFG%
 echo SKEL_LIB          = %SKEL_LIB%
 echo SKEL_DATA         = %SKEL_DATA%
+echo SKEL_DOC          = %SKEL_DOC%
 echo SKEL_RESULTS      = %SKEL_RESULTS%
 echo SKEL_LOG          = %SKEL_LOG%
 echo SKEL_EXAMPLES     = %SKEL_EXAMPLES%