fulcio-server: add html page when humans reach the server via the browser

[cpanato]

When we access via browser the Fulcio server (for example) https://fulcio.sigstore.dev/ it returns

{
  "code": 404,
  "message": "path / was not found"
}

maybe is good to add a webpage and say something :) instead of return a json 404.

This PR adds a very simple homepage when humans access that page

if we don't want that and this is by design feel free to close this PR

[bobcallaway]

I wouldn't touch the openapi file at all for this; we can handle it by declaring the middleware handler directly for the "/" path.

[cpanato]

thanks @bobcallaway for your review and insights! really appreciate
PTAL

[dlorenc]

LGTM now, I'll wait for Bob.