Add a link checker CI (and justfile?)

[nuke-web3]

ahhhh gross. I quite strongly oppose this syntax for that reason & opt to link the first reference in a page and omit latter on rather than link-ify all the things.

Do we not have a CI link checker setup yet? This should be not hard to do if not - example using https://lib.rs/crates/mlc:

• https://github.com/nuke-web3/book/blob/main/.github/workflows/checks.yml
• https://github.com/nuke-web3/book/blob/main/justfile#L134-L140

also relevant

• https://adventures.michaelfbryan.com/posts/linkchecker/
• ⭐ https://github.com/lycheeverse/lychee

Originally posted by @nuke-web3 in #116 (comment)