Close down spotlights and announce transfer to RSD

Since the latest update of the RSD, we're close to being able to clean out the SW spotlights from hifis.net entirely.

To-dos:

• finetune display of spotlights in the top banner of the RSD browse?
• shut down spotlights in hifis.net
  • _spotlights
  • spotlights.md
  • all dependencies in _layouts, _sass?
• add a redirect to RSD from hifis.net/spotlights
• update menu in nav bar
  • either leaving only the title "Use cases" in the 5th category
  • or moving "Community Use Cases" to another category, e.g. "Latest & Media"
• close down issues #323 (closed) #352 (closed) #428 (closed)
• close down MRs !503 (closed) !504 (closed) !601 (closed)
• create a blog post to announce the transfer and encourage people to register their SW to RSD, maybe even explain how spotlights are selected? and mention the software award (I guess the winners will be spotlights)

@christian.meessen I think you told @jandt-desy that @beate.hetenyi would take care of this so I added you as assignee. But please let me know if you want help, I can help.