I saw Scott Hanselman's post on using your own domain for Mastodon discoverability.

Instead of adding a webfinger file, I added this redirect on Netlify:

/.well-known/webfinger https://ruby.social/.well-known/webfinger?resource=acct:scottw@ruby.social

Longer term, there is likely some benefit to hosting the file yourself, but this gets you rolling quickly today.