Update URL to Macaulay2 webpage in docs

[d-torrance]

Now that https://faculty.math.illinois.edu/ doesn't exist anymore, we fix various links in the documentation, either using local links (for packages referring to other packages) or replacing with https://macaulay2.com/.

@DanGrayson -- do you have an updated URL to replace https://faculty.math.illinois.edu/~dan/? Or should we just remove the HomePage from your packages?

[d-torrance]

The HTML5 validation check I wrote is failing on this and some other PR's. I'll see if I can figure out what's happening.

[d-torrance]

I noticed a bunch more links that used math.uiuc.edu (which had apparently been forwarded to faculty.math.illinois.edu), so now they've been updated as well.

[mikestillman]

@d-torrance This seems to fail still? Do you see the problem?

[d-torrance]

@d-torrance This seems to fail still? Do you see the problem?

Weird! It worked locally... The error is a bit different now. I'll play around with it some more.