Add "local deployment" instructions

[apoelstra]

This is important for testing at the very least..

[delta1]

The readme explains how to do that with Nix (and Docker), do we want instructions for doing without those?

[apoelstra]

I would appreciate if the nix instructions explained how to run the thing (and if they didn't use flakes). But yeah, we should have straight command-line instructions for people who aren't using nix or docker.

Looks like it's just trunk serve. Okay, nice. I have a user report that he tried this and the site "didn't work". Just a blank screen indicative of a panic in wasm.

We should also update the fix-links.sh script to make it more generic, and/or make it a build-time setting. But that's independent of this issue.