Review and update the linux installation instructions

Let's review and update instructions to download and install Tor Browser on Linux.

Remove the screenshot of the Nautilus file manager. It's outdated.
Try to simplify the instructions, and potentially move some parts (for instance, changing the permissions of start-tor-browser.desktop isn't a necessary/less likely step and since we are shipping a tar file very likely something user's might not have to deal with) to manual/troubleshooting