potential breaking change with baseDir-option

> I'm concerned this is a breaking change in behaviour and needs to be highlighted as such when it gets published - but would like a second opinion.
>
> Prior to this change, if you set `baseDir`, the swagger static content would be served from the `static` folder included with this module. You could then set `baseDir` to point at another directory of content that should served in addition.
>
> With this change, if you set `baseDir` it expects you to have copied the `static` folder included with this module into your `baseDir`.
>
> So a working configuration prior to this PR will no longer work when this PR is applied.
>

_Originally posted by @knolleary in https://github.com/fastify/fastify-swagger-ui/issues/108#issuecomment-1887149064_