Use VERCEL_URL if available for the base url in sitemap.xml and robots.txt

[nicholeuf]

As of #17 , we require the base url for the sitemap.xml and robots.txt files.

Vercel should provide the actual generated deployment URL and it would be nice to use it rather than a hard-coded environment variable in the @/app/lib/getBaseUrl.ts file. From the documentation:

The domain name of the generated deployment URL. Example: *.vercel.app. The value does not include the protocol scheme https://

Links:
https://vercel.com/docs/projects/environment-variables/system-environment-variables#system-environment-variables