Doc Serve Regression Test and Fix

[epiro-ps]

We'll want to add a regression test that reproduces the following issue and push a fix for it.

[evan@tower:~/Repos/dUSD]$ nix run .#offchain:docs:serve
error: Invalid value for '<root>': No such file or directory (os error 2)

Perhaps we could add the test into the flake check for the offchain:docs:serve app. We could trigger the serve command, wait for the doc server to come up, curl the endpoint, and assert that it is the expected page. Then CI will fail if this occurs again.

[Geometer1729]

forgot to mark this to be closed by #345