Add PHEP 3 to PyHC's Website

[sapols]

Now that PHEP 3 has been formally accepted and released, this PR does a few things:

1. Adds PHEP 3 to our PHEPs page
2. Creates a (placeholder) PHEP 3 subpage for displaying the support schedule—required by PHEP 3's "How to Teach This" section (I'll implement the page soon)
3. Makes a blog post announcing PHEP 3's adoption

I think this is ready to merge. But I'll wait a bit in case you have feedback on the blog post text or where I put the new subpage, @jibarnum @jtniehof

[sapols]

FYI here are what things look like rendered: