Let MkDocs clean previously generated HTML pages

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

Dockerfile

@@ -5,7 +5,7 @@ FROM python:3-alpine as docs
 ADD . /usr/src/app/shaarli
 RUN cd /usr/src/app/shaarli \
     && pip install --no-cache-dir mkdocs \
-    && mkdocs build
+    && mkdocs build --clean
 
 # Stage 2:
 # - Resolve PHP dependencies with Composer

Makefile

@@ -207,7 +207,7 @@ htmldoc:
 	python3 -m venv venv/
 	bash -c 'source venv/bin/activate; \
 	pip install mkdocs; \
-	mkdocs build'
+	mkdocs build --clean'
 	find doc/html/ -type f -exec chmod a-x '{}' \;
 	rm -r venv