Add build docs script

Backport elastic#1873

Makefile

@@ -34,3 +34,8 @@ fmt:
 
 simplify:
 	$(foreach var,$(PROJECTS),make -C $(var) simplify || exit 1;)
+
+# Builds the documents for each beat
+.PHONY: docs
+docs:
+	sh libbeat/scripts/build_docs.sh ${PROJECTS}

libbeat/scripts/build_docs.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# Checks if docs clone already exists
+if [ ! -d "build/docs" ]; then
+    # Only head is cloned
+    git clone --depth=1 https://github.com/elastic/docs.git build/docs
+else
+    echo "build/docs already exists. Not cloning."
+fi
+
+# beatnames must be passed as parameters. Example: packetbeat filebeat
+for name in $*
+do
+  index="$GOPATH/src/github.com/elastic/beats/${name}/docs/index.asciidoc"
+  echo $index
+  if [ -f "$index" ]; then
+    echo "Building docs for ${name}..."
+    dest_dir="build/html_docs/${name}"
+    mkdir -p "$dest_dir"
+    ./build/docs/build_docs.pl --doc "$index" -out "$dest_dir"
+  fi
+done