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.sh

@@ -15,6 +15,7 @@ export SPS_SKEL="$(cd "$SCRIPT_DIR/.." && pwd)"
 export SKEL_BIN="$SPS_SKEL/bin"
 export SKEL_LIB="$SPS_SKEL/lib"
 export SKEL_DATA="$SPS_SKEL/data"
+export SKEL_DOC="$SPS_SKEL/doc"
 export SKEL_CFG="$SPS_SKEL/cfg"
 export SKEL_LOG="$SPS_SKEL/log"
 export SKEL_TESTS="$SPS_SKEL/tests"
@@ -38,6 +39,7 @@ 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"