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:

[jibarnum]

Sorry @sapols for my delay - holiday break! This looks great to me. I'd say go ahead and merge unless @jtniehof has any further comments.

[jtniehof]

No comments from me!

[sapols]

Thanks guys! I also realized we needed to update the Grading Guidelines and the Review Template to reflect the new PHEP 3 rules instead of the old Python3 rules. I just pushed those changes.

I'm gonna merge now. If you have qualms with the new wording in those instructions I'll happily make changes. But they're straightforward enough that I assume you probably won't :)