Update www.torproject.org links (closes: #17717)

This MR updates links to www.torproject.org.

More specifically, this MR completes the work listed in #17717 (comment 152083):

• Features
  • • https://www.torproject.org/projects/torbrowser.html.en → https://www.torproject.org/
    • Nowadays, the torproject.org website is so focused on Tor Browser that we should point directly to their homepage.
  • • https://www.torproject.org/docs/torbutton/
    • Torbutton is not a thing anymore for users (c31b2531) so I think that we should:
      • Remove this button point
      • Rephrase the FAQ that mentions Torbutton
      • Remove the #torbutton anchor in our Tor Browser doc
• Fingerprint
  • • https://www.torproject.org/docs/faq#EntryGuards → https://support.torproject.org/tbb/tbb-2/
• Warnings
  • • https://www.torproject.org/docs/faq.html.en#CanExitNodesEavesdrop → https://support.torproject.org/https/https-1/
  • • https://check.torproject.org/cgi-bin/TorBulkExitList.py → https://metrics.torproject.org/exonerator.html
  • • https://www.torproject.org/projects/torbrowser/design/: OK!
• Pidgin + Thunderbird
  • • Tor is moving from Trac to GitLab and I'm not sure these will continue to work but let's ignore them for now.
• Bridge mode
  • • https://www.torproject.org/docs/bridges
    • OK, though I'm tempted to remove these 2 sentences.
  • • https://www.torproject.org/docs/bridges#PluggableTransports
    • Remove and mention explicitely obfs2. ScrambleSuit and Obfs2 are not a thing anymore. Obfs3 is so little used that I'm not sure that it's protection is useful anymore: https://metrics.torproject.org/userstats-bridge-transport.html?start=2020-04-04&end=2020-07-03&transport=!%3COR%3E&transport=obfs2&transport=obfs3&transport=obfs4&transport=scramblesuit
• Download
  • • https://www.torproject.org/download/download-easy.html: OK!
• Press
  • • https://www.torproject.org/about/overview.html.en
    • I would remove the entire bullet point.
• News
  • • It's not worth updating links in very old news. I usually remove the link but keep the text.
• FAQ
  • • https://www.torproject.org/docs/faq.html.en#ChooseEntryExit → https://support.torproject.org/tbb/tbb-16/

Closes: #17717 (closed)