Automate build of user-guide on CI

[michael-okeefe]

The user guide should be automatically built by the CI (Github Actions) ideally.

We would only need to use an Ubuntu instance. The current build files are in docs/user-guide. Please see the dependencies in docs/user-guide/README.md for guidance on how to set up the system to build.

Some investigation will be required to see if Github allows us to store artifacts (the built PDF) somewhere when using private repositories without a team or enterprise license.

At a later time, we will investigate building the HTML documentation but not yet.