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

[wilfwilson]

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

[stertooy]

Good idea.

In addition to that, maybe we should also mention that package authors may have to setup the authentication? E.g. something like

You may also have to create an authentication token and add it to the codecov-action inputs (token: ${{ secrets.CODECOV_TOKEN }}), see the documention here.

[wilfwilson]

Sounds good.

[fingolfin]

All good ideas