Enforce use of "en/latest" in cross-links

[nomirosa]

it is going to be very easy to accidentally cross-link to a document using the URL that appears in the browser when you visit it. this URL is typically going to include the version information, e.g.:

https://crate.io/docs/crate/reference/en/4.2/sql/statements/select.html

all internal links in our docs should use "en/latest" instead of linking to a specific version. so the above link should be changed to this:

https://crate.io/docs/crate/reference/en/latest/sql/statements/select.html

we should be able to lint for this