doc: Fix nixos download page links

Those two links are currently broken, update them to point to the
up-to-date nixos download page.

Changed files
+4 -4
nixos
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nixos/doc/manual/installation/installing-virtualbox-guest.section.md
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Installing NixOS into a VirtualBox guest is convenient for users who
want to try NixOS without installing it on bare metal. If you want to
use a pre-made VirtualBox appliance, it is available at [the downloads
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page](https://nixos.org/nixos/download.html). If you want to set up a
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VirtualBox guest manually, follow these instructions:
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page](https://nixos.org/download/#nixos-virtualbox). If you want to set
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up a VirtualBox guest manually, follow these instructions:
1. Add a New Machine in VirtualBox with OS Type "Linux / Other Linux"
+2 -2
nixos/doc/manual/installation/upgrading.chapter.md
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contains the channel's latest version and includes ISO images and
VirtualBox appliances.) Please note that during the release process,
channels that are not yet released will be present here as well. See the
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Getting NixOS page <https://nixos.org/nixos/download.html> to find the
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newest supported stable release.
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Getting NixOS page <https://nixos.org/download/> to find the newest
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supported stable release.
When you first install NixOS, you're automatically subscribed to the
NixOS channel that corresponds to your installation source. For