docs: Made README links absolute

[alexschimpf]

Pull Request Checklist

• [X] Make sure to read the contributor guide here.
• [X] Make sure you are making a pull request against the main branch.
• [X] Make sure your pull request title matches the requested format.
• [X] Make sure to lint, type-check, and run unit and API tests.

Description

README.md is also used as PyPI description. So links need to be absolute to work there, too.

[alexschimpf]

:tada: This PR is included in version 4.0.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: