Add gh-pages build step.

.gitignore

@@ -17,3 +17,7 @@ scratch.wren
 
 # The baseline file is machine-specific, so doesn't get checked in.
 benchmark/baseline.txt
+
+# built docs get copied here, which is presumed to be a separate checkout of
+# the repo so they can be pushed to GitHub Pages.
+gh-pages/

Makefile

@@ -60,3 +60,7 @@ docs:
 # Continuously generate the Wren site.
 watchdocs:
 	@./script/generate_docs.py --watch
+
+# Build the docs and copy them to a local "gh-pages" directory.
+gh-pages: docs
+	cp -r build/docs/. gh-pages