Add shell script to build docs

Useful for testing docs without changing BUILD_DOCS if it's set to OFF.

docs/build.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+# usage:
+# ./build.sh
+# ./build.sh sphinx-executable
+# JOBS=3 ./build.sh ...
+
+cd $(dirname "$0")
+cd ..
+
+sphinx=sphinx-build
+if [ -n "$1" ]; then
+    sphinx=$1
+fi
+
+if [ -z "$JOBS" ]; then
+    JOBS=2
+fi
+
+"$sphinx" -a -E -q -b html . ./docs/html -w ./docs/_sphinx-warnings.txt -j "$JOBS"