Split website generate-release target in two

[cperciva]

Summary

Turn this into a generate-release target and a generate-hardware-notes
target, so I can add an upcoming release to the website and fill in
the hardware notes after the relevant releng/X.Y branch exists.

[cperciva]

I didn't realize that the generate-release target existed when I was putting together the website bits for 13.3, but I'd like to use it for 14.1.

[cperciva]

Fix typo