Add mdbook-linkcheck to CI

[jeckersb]

https://github.com/Michael-F-Bryan/mdbook-linkcheck

I've been catching these manually[1], but unsurprisingly computers are better at parsing things than my fallible biological parser.

[1] https://github.com/containers/bootc/pull/520/files/615ead5c9cc85066ef07af1da5321d1c112acc4f#diff-e0b4a488eccbd4e71db5387b0916834520b83da8da89b17f1842760ffd5e1963

[cgwalters]

OK I had to revert this as it broke the site. I only dug in a bit but I think the problem is that the linkcheck run seems to add a new top-level linkcheck/ directory in the output paralleling the html/ directory from the main run.

We could probably just remove it. Or, maybe we don't need to run link check on the main build?

Needs some investigation and testing.

[jeckersb]

OK I had to revert this as it broke the site. I only dug in a bit but I think the problem is that the linkcheck run seems to add a new top-level linkcheck/ directory in the output paralleling the html/ directory from the main run.

We could probably just remove it. Or, maybe we don't need to run link check on the main build?

Needs some investigation and testing.

Of course I just read this after I wrote #533 (comment)

So yes, that's the problem. We might as well run linkcheck again on the main build, but we just need to upload the artifacts correctly with the right path.