Treat warnings as errors in doc building action

[sfmig]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
On issue #252, @lochhh pointed out that sphinx-build returns a successful exit code (0) even if it encountered errors.

This can be quite annoying: in practice it means we need to remember to manually check the logs of the action (or locally), to confirm that there were no errors, even if the action shows a pass.

The simplest workaround seems to add the flags -W --keep-going so that sphinx-build runs to completion, treats warnings as errors, and returns a non-zero exit code if there are errors or warnings.

What does this PR do?
Adds the flags -W --keep-going to the sphinx-build in the doc building workflow file.

References

In movement we build the docs in CI using make, and have a PR to add the -W --keep-going to the makefile:

• issue #252.
• PR #256, where we added this flags to the Makefile.

How has this PR been tested?

I have checked that the syntax of the sphinx-build command works locally as expected. I am not sure how to test this further.

Is this a breaking change?

I think no.

Does this PR require an update to the documentation?

AFAIK this is not documented beyond the workflow file itself.

Checklist:

• [ n/a ] The code has been tested locally
• [ n/a ] Tests have been added to cover all new functionality
• [ n/a ] The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit