chore: move gh-pages output directory under build/ and update .gitignore

The gh-pages directory is now created inside build/ instead of at the project root.
The .gitignore entry for the old gh-pages location is removed, and the Makefile
cp target is updated from gh-pages to build/gh-pages.

.gitignore

@@ -17,7 +17,3 @@ 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/