x/pkgsite: WTFPL Is not recognized

[azer]

I use WTFPL in nearly all my packages, and pkg.go.dev website doesn't show documentation due to its whitelisted license list missing WTFPL.

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/azer/crud/v2#section-documentation

What did you expect to see?

Documentation of my WTFPL-licensed package

What did you see instead?

Error: Documentation not displayed due to license restrictions.

See our license policy.

[seankhliao]

See #36840 (comment)