ci(build): Build book on main branch as well

In order to allow deployments of the development version of the book, the book needs to get built on main as well.

Current plan:

• deploy main as the draft book
• deploy publish as the release version of the book

We'll use webhooks to notify the server of the new book version. The server will then just download the artifact that was/is generated during the build-book stage of the pipeline.

The server is pretty much ready, we only need CI changes and the webhook itself.

.gitlab-ci.yml

@@ -36,6 +36,7 @@ build-book:
   stage: build
   only:
     - publish
+    - main
   script:
     - pip install -r requirements.txt
     - jupyter-book clean book/