Print URL after building docs (#1945)

This makes it easier to open the generated HTML docs. In some terminal
emulators, it allows the user to click the link instead of having to
navigate to the build directory in the web browser.

Author: srittau

docs/Makefile

@@ -29,6 +29,8 @@ build:
 html: BUILDER = html
 html: build
 	@echo "Build finished. The HTML pages are in build/html."
+	@echo "file://$(shell pwd)/build/html"
+	@echo
 
 text: BUILDER = text
 text: build