Tweak npm scripts

* add `--strict` in the mkdocs build like on CI
* add a `start` script to run serve (can be used with `npm start`)
* move `build` to `pretest`

As a result, build runs once on CI now

.github/workflows/ci.yml

@@ -40,8 +40,5 @@ jobs:
       - name: Install npm dependencies
         run: npm ci
 
-      - name: Build docs
-        run: mkdocs build --strict
-
-      - name: Test
+      - name: Build and test
         run: npm test