update social media links in order to promote free networks more

TODO:

• replace mastodon, twitter, reddit links in the navigation with one "social media" button. Link it to /social
  • as separate commit, then apply it to the podcast page too
• create that page, put the free software networks on top (mastodon, lemmy) and the other ones below with some disclaimer text
• change comment links in each blog post to promote the free networks. If we have posted an article to both, then let's put the proprietary networks in <small> or something.
  • maybe later / maybe with the new blog posts. the new "chats & social media" has a huge disclaimer about proprietary platforms now, that should get the point across.