Format incipient links differently (may obviate 404)

[peterkaminski]

When a link goes to a page that doesn't exist, format it differently -- maybe with a wiggly underline, purple or red underline, a different background color, etc.

It might be left as a link, or just formatted to look similar to a link, but not actually be clickable to go anywhere.

There might be a tooltip popup that tells the user this is an incipient link.

This interacts with the custom of using 404.md for Netlify-deployed wikis, in which the 404 page can explain about uncreated pages in wiki culture, e.g.:

# Page Doesn't Exist Yet (404)

<!-- This page is a special page for the website version. It is not a regular wiki page. -->

<!-- If you can see this notice, it is okay to ignore this page. Or, if you want to change the 404 message on the website version, go ahead and edit this page. -->

## But Read On!

You have followed a link to a page that has not yet been created.

On a regular website, this indicates a problem.

On a wiki, it's not a problem; it just means that a wiki author made a link to a page that _will_ exist in the future, but it is not yet that future time. :-)

**You can just go back to the previous page in your browser; or, go to the [[README|Home]] page of this website.**

If you'd like, you can contact MAINTAINERS and let them know you'd like to see the page created. Let them know which link you followed.

Thank you for browsing the YOUR_WIKI_NAME wiki. :-)