Restructure the download page and "Get Tails" documentation

• Make the download page shorter, especially at the end.
  • Create separate pages that can be partly inlined from the download page for:
    • Verify the ISO image
    • Install or upgrade Tails
    • Stay tuned
    • Start Tails!
  • Resolve the ambiguity of having installation and boot instructions both in the “Get Tails” and “First steps with Tails” sections. Maybe create a new section “Installation and upgrade”.

Related issues

• Is duplicate of #8580 (closed)

Original created by @sajolida on 6321 (Redmine)