Build link-checking for book

[skade]

Land #61 and make sure it doesn't break netlify ;).

• Turn book building into a script
• Make sure link checking only triggers during testing
• On deploy environments, download the mdbook and linkcheck binary

[yoshuawuyts]

@skade can this be closed?

[yoshuawuyts]

Ah, actually going to close this as a duplicate of #82 -- even though it's filed later it seems to still be active.