Improved rss readme (#12157)

* docs: improved rss readme

* chore: changeset

* fix: remove empty title

Co-authored-by: Armand Philippot <git@armand.philippot.eu>

* edit: slash wording

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* edit: installation and use

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* edit: see the guide

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* edit: remove confusing guide link

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* edit: clarify what enclosure is used for

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* edit: remove description vs. content blurb

* new: expand xmlns guide to explain usage

---------

Co-authored-by: Armand Philippot <git@armand.philippot.eu>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

.changeset/silly-tomatoes-kick.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/rss': patch
+---
+
+Improves README configuration reference.