Make dead links warnings

[norswap]

We're auto-generating some of our docs, including bringing typedoc-generated docs into our vocs site. Unfortunately, this can create dead links that are outside our control, causing the build to fail.

I can also see this being problematic if the docs site is supposed to link out to other pages that are generated or deployed separately.

Funnily enough, the one case comes from Viem, as we return a Viem config from a function and typedoc pulls in the entire documentation for the return value. The offending link is a relative link in the docs of getFilterChanges. All the other links we inherit from the Viem docs are prefixed with https://viem.sh and don't cause issues.

The simple fix here would be to make dead links a warning instead of an error, or to have an option to control the behaviour.

[norswap]

Of note, I've also just noticed that deadlink detection only applies to link of the form [xxx](https://x.com), however reference links (see below) escape detection:

hello [xxx]

[xxx]: https://x.com