Add Atom feed using the jekyll-feed plugin

[ptgolden]

This adds an Atom XML feed for content under _posts using jekyl-feed. No additional install steps should be necessary since that plugin is pre-installed for GitHub Pages and the Jekyll Docker image.

[jsstevenson]

👍

I might suggest the disable_in_development setting because it looks like feed generation adds about a second to build time (obviously that's not too bad, but it can add up if you're making repeated edits), unless there's a good reason not to

[matentzn]

@ptgolden do you want to add the flag @jsstevenson suggests?

[ptgolden]

Done 👍

[nlharris]

Can this be merged yet?

[jsstevenson]

(should be fine to merge imo)

[matentzn]

Let's try it