docs(build): switch from html to dirhtml builder for clean URLs

why: The html builder generates flat .html files, but the CloudFront
function now redirects .html -> /. Without dirhtml output (which creates
directory-style index.html files), the redirects lead to 403s because
the target paths don't exist in S3.
what:
- Change html, start, and design targets to use -b dirhtml
- Produces quickstart/index.html instead of quickstart.html
- CloudFront serves /quickstart/ -> quickstart/index.html correctly

Author: tony

docs/Makefile

@@ -45,7 +45,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
@@ -182,8 +182,8 @@ dev:
 	$(MAKE) -j watch serve
 
 start:
-	uv run sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) --port ${HTTP_PORT} $(O)
+	uv run sphinx-autobuild -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) --port ${HTTP_PORT} $(O)
 
 design:
 	# This adds additional watch directories (for _static file changes) and disable incremental builds
-	uv run sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) --port ${HTTP_PORT} --watch "." -a $(O)
+	uv run sphinx-autobuild -b dirhtml "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) --port ${HTTP_PORT} --watch "." -a $(O)