Updated RSS documentation

[rkingett]

Made several changes to the RSS documentation.

I couldn't help it, in the, email to RSS documentation, I did a tad bit of self promotion because I could use the PR but partially because I couldn't quite find a good enough example to use from one of Buttondowns newsletters.

If you want to change it, at least change it to the evergreen buttondown.com/weird writer link.

I added some more digest example, used more variables in the example given so people could see how they work, and cleaned up minor formatting.

In the, RSS to email, I couldn't get the

to render as an ordered list with the screen reader. Every
• was an item 1, a list item with 1 item instead of 1 list item with many items, like 10 items, so triple check that code before merging.

  Cleaned up minor formatting.