[e.cash] add rss feed for blog

Summary:
Adding a rss generator package and function to make rss feed for blog as requested
fyi the reason the generateRssFeed function is where it is, is because next was throwing errors when trying to use fs outside of a getStaticProps file

Test Plan:
preview the site and check out /rss.xml. Should see rss data
Can also use a rss reader like this one to test https://chromewebstore.google.com/detail/rss-feed-reader/pnjaodmkngahhkoihejjehlcdlnohgmp
Plug in localhost if youre running locally, or the bot url and see if the feed populates correctly

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15318