Add escalation of warnings to errors in the build of the Sphinx docs.

[Andrew Coughtrie (andrewcoughtrie)]

Description

I've added flags to the build-docs workflow to catch build warnings produced by the Sphinx build. The flags do the following:

Flag	Purpose
-W	Turn warnings into errors.
--keep-going	Run to completion and exits with exit status 1 if errors are encountered.
-n	Run in nitpicky mode. Currently, this generates warnings for all missing references

There is also a small fix to the workflow syntax to make sure the branch checked out by the workflow is the submitting ref (e.g. the branch for the PR or main on merge)

Linked issues

Closes #154

Developer tags

Type of change

• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration.

• New tests have been added.
• Tests have been modified to accommodate this change.
• GitHub workflows have been changed or added.

Checklist:

• The change has been checked for design compliance by an experienced SSE
  
• My code follows the style guidelines of this project.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes, for both debug and optimised builds.
• No AI tools have been used in the creation of this change.

Reasoning on why any of the above boxes have not been checked

Review Checks (To be filled in by the reviewer/s)

• Has the developer completed the appropriate sections above?
• Is the change compliant with LFRic Core principles?
• Is the testing coverage sufficient?
• Have any technical debt workarounds identified had issues opened and interested parties notified?

[mo-lucy-gordon]

This looks like a good change, can't see any issues