Provide different feeds for books and blog entries

[mliebelt]

The current provided feed is not ok

• The entries without date the are the ones about categories, and authors. So they should be excluded from the RSS feed.
• Most of the entries are about books, they should have their own feed (that is visible to feed readers, of course)
• The few blog entries should be separated from that.

Gatsby allows to have different feeds, but to do that, I have to understand the configuration (which is now just the default).