Fallback to package project URL when failing to resolve help xref

[glopesdev]

Most built-in package documentation is built on docfx which provides a xrefmap.yml file to resolve class UIDs to URLs. However, for simpler projects or projects under development it would be convenient to have as a fallback the specified package project URL of the package.

This would allow people using the package to open a developer specified URL where they might be able to learn more about the package even before a full docfx website is available.

This fallback could also be activated if there is a xrefmap.yml file but it is somehow outdated so that at least the documentation landing page is launched.