build: replace Hugo targets with MyST in Makefile

- html: hugo → myst build --html (output now in _build/html/)
- html-all: copy _build/html/ to public/ then build external content
- serve: hugo server → myst start
- clean: also removes _build/

Author: lundybernard

Makefile

@@ -24,17 +24,19 @@ cookie: cookie_ruby_deps cookie_web_prepare
 
 external: cookie
 
-html: ## Build learn site (without external content) in `./public`
+html: ## Build learn site in `./_build/html`
 html: prepare
-	hugo
+	myst build --html
 
-html-all: ## Buildlearn site (with external content) in `./public`
-html-all: html external
+html-all: ## Build learn site with external content in `./public`
+html-all: html
+	cp -r _build/html/. public/
+	$(MAKE) external
 
-serve: ## Serve site, typically on http://localhost:1313
+serve: ## Serve site, typically on http://localhost:3000
 serve: prepare
-	@hugo --printI18nWarnings server
+	myst start
 
 clean: ## Remove built files
 clean:
-	rm -rf public
+	rm -rf _build public