fix(addon-docs): assume links starting with "https://" are external

[GuilleDF]

Issue: #17308

What I did

• I updated the check for MDX links to see if the link starts with https://. Not all external links will, but this seems like a pretty easy way to detect most cases.
• I set target="_blank" as default for external links, but it can be overridden by the user. Let me know if you agree with this, or would prefer not to have this.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

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[GuilleDF]

Not sure about the test failures, they do seem to pass on my local machine

[kylegach]

Hi, @GuilleDF. Thanks for your PR and congratulations on your first contribution! I left a detailed comment which needs addressed, then this should be good-to-go.

[kylegach]

This is great! Thanks again, @GuilleDF.