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- /index and /about were serious mistakes!
- /{install,upgrade}/*/*-overview.mdwn should be indexed for people
  searching for "install Tails from Windows" for example.
- /install/download can be indexed as it provide links to the different
  installation and upgrade scenarios.
- /install/download-iso shouldn't rather not be indexed as we don't want
  most people to use it without understanding that it's only for DVDs
  and VMs.

I discovered this today!

This advanced warning breaks the linear flow of the instructions.

The Shift+Restart technique on Windows should make it less useful
anyway.

- It will be useful for people search for this on our website.

- It makes it easier for Help desk to point to the relevant
  troubleshooting sections. (#14788)

- Thanks to the Shift+Restart technique on Windows, the Boot Menu key
  should only be used by Linux users now. Their computer should already
  have a minimum configuration to start on USB stick and they less
  likely need to edit their BIOS settings.

People might be scared a bit by the instructions. Displaying the
animation first gives it more visibility and might help people reading
the instructions afterwards.

It's only adding noise here.

Closes #17815

This reverts commit 780b23cf.

Upstream gave up on the feature we were waiting for.

This reverts commit 25d8a66b