This adds a new workflow for building the Gluon documentation.

In contrast to the existing build workflow, the documentation
is built for every branch, as it only takes around two minutes
to render the HTML documentation.

- Rely on shebang instead of setting shell in workflow
- Run whole install-dependencies.sh in sudo
- Use /bin/sh instead of bash
- set -e

The new step is added with `if: ${{ !cancelled() }}`, so the logs are
stored even when the build fails.

This enables build-testing only on master as well as next and release
branches.