Add automated check for remote links in skill contents

The automated check for remote links has the following purposes:

1. Make sure all the remote links have trusted host names, such as learn.microsoft.com, github.com, etc. This is intend to catch injection attacks.
2. Make sure all the remote links are still valid (e.g. not getting 404).
3. Warn the author when the link follows certain problematic patterns. For example, referencing a github source code location or a release artifact without a pinned version is not a good practice. They are subject to frequent changes that may invalidate the link.