Recommend using codecov/codecov-action with the input fail_ci_if_error: true #24

I recently noticed that the Semigroups and Digraphs packages had been failing to upload coverage data to Codecov for a very long time because of an authentication error. However, all of the CI related to uploading coverage was passing, so we did not realise.

I'm going to add the input fail_ci_if_error: true (the default is false) to my workflows, and I think it would probably be best practice to recommend this to users both in the README here and (yes, I know it's a separate repo) probably in the Example package, and elsewhere that we try to encourage best practice.

Otherwise, the codecov/codecov-action step might be silently failing for package developers, and they could be none the wiser.

Here's the documentation describing the arguments to this action: https://github.com/codecov/codecov-action?tab=readme-ov-file#arguments