Add test to check that all URLs we use are alive

In https://github.com/nexB/vulnerablecode/actions/runs/8817758473/job/24204933651?pr=1466#step:6:145
we had this error:

> Warning, treated as error:
> /home/runner/work/vulnerablecode/vulnerablecode/docs/source/importers_link.rst:30:broken link: https://www.openssl.org/news/vulnerabilities.xml (404 Client Error: Not Found for url: https://www.openssl.org/news/vulnerabilities.xml)
> ( importers_link: line 30)
> Error: Process completed with exit code 2.

There is also some related discussion to OpenSSL in https://github.com/nexB/vulnerablecode/issues/1128

We should have a daily CI check that systematically validates that all the root/seed URLs we use in the code are alive and well and fails loudly otherwise.