Add RSS feed for all boards

[tannewt]

For @ptorrone

[makermelissa]

I submitted PR #440 for this and used the url circuitpython.org/feed.rss, but if we want it different than that, I can change it to whatever. It's dependent on the PR #439 being merged first as @jwcooper had mentioned in #299.

[makermelissa]

Done!