In #2153 I aimed to fix a link but broke it.
In this PR, I fix it and add
[`mdbook-linkcheck`](https://github.com/Michael-F-Bryan/mdbook-linkcheck)
to avoid future cases.

Some past fixes that could have been prevented, in addition to mine in
this PR:
* #811
* #2064
* #2146

Note:  
`mdbook-linkcheck` may also check external links with a configuration
change.
It can be beneficial to check also external links from time to time. I
ran it here and found 3 broken links.

Maintainers - sorry for the lack of a preceding issue. We can discuss it
here.
Some remaining work is to fix the outdated internal links in the
translations, not done here.
Let me know what you think about the proposed contribution.

This PR completes #1502.