Support for 'ignore' list in the URL checker

[epijim]

Do we support a an 'ignore' list for URLs?

Based on here I don't think so: https://github.com/phuse-org/devops-toolkit/blob/main/.github/workflows/urlchecker.yml#L47

Assuming it's not supported - I've removed the comment about lycheeignore currently in the site, which I assume is an old dependency we don't use now.

[cicdguy]

I'd argue that there shouldn't be an ignore list at all. When CRAN investigates URLs, they don't ignore certain URLs or ask authors which URLs they should ignore. If there are invalid or non-conformant URLs, they should be simply removed from the packages.

[bms63]

We have a slack 502 link that CRAN lets by, but I've been wondering if there is another way to have the slack link display in our README.

I've noticed that CRAN lets things by and then wallops you later!!