website/press: Adjust all links

• Fix RSS/Feed links
  
  They were without the anchor part, causing 404 requests: https://www.freebsd.org/press/2021-03:0 The correct is: https://www.freebsd.org/press/#2021-03:0

• Make links unique
  
  When investigating the above issue, we could note that the links were not unique. Every time a new press item is added, the entire chain of links is changed.
  
  We add the old behavior with this change, which counts press items for each month and generates the unique link based on the month and in the month counter.
  
  This change also needs to be added to the News and Events items.

Reviewed by: carlavilla
Differential Revision: https://reviews.freebsd.org/D31190