Added in linkchecks and coverage checks for gha ci

[ESadek-MO]

Closes #340.
Finishes work started in #415.

[ESadek-MO]

Readded in a couple of links to be ignored:

https://software.ecmwf.int/ is no longer correct, https://confluence.ecmwf.int/ is, however it doesn't seem to have a landing page for GRIB.

I'm ignoring any errors in release notes atm, as it seems questionable to maintain that, but that might want to be discussed.

[stephenworsley]

Looks good.

[bjlittle]

@ESadek-MO @stephenworsley

The coverage is not completed nor working

[stephenworsley]

@bjlittle Is it just not running in the CI at all then? I've had coverage problems in iris-esmf-regrid also, but they've been causing CI failures.

[bjlittle]

Seems like the --coverage flag is passed into nox from the GHA, but then nothing is done with it.

[bjlittle]

Also, if it was working, codecov would add comments to the pr.

I suspect tokens require to be also configured.

[stephenworsley]

yeah, I suspect I'll need to sort out tokens for iris-esmf-regrid also

[pp-mo]

Also, if it was working, codecov would add comments to the pr.

I suspect tokens require to be also configured.

Not sure I follow exactly (I'm a bit rusty on this), but I think we can get around to this in due course.
@bjlittle @stephenworsley @ESadek-MO Can you please create a follow-on issue explaining what needs doing here ?