fix!: move justfile to standardized, non-conflicting new location

- The old location was conflicting with upstream images (main, nvidia, etc), and was causing the file to be overwritten.

- It was therefore decided that each image should prefix their own justfile names, to avoid clobbering, to easily allow image makers to bundle multiple "modular" justfiles, and to allow end-users to easily include the particular modules they want.

- The name `custom.just` represents a "template name" for this "custom uBlue image", while being neutral enough to use long-term (unlike the alternative name `startingpoint.just`, which doesn't flow nicely).

- All redundant commands that already existed upstream in `ublue-os/main` have been removed, to follow the new "modular inclusion" nature of uBlue's "justfile" organization, which also means that we'll never have to manually update it to match upstream anymore. No more duplicated effort! ;)

- Updated README instructions to mention the new way of including justfiles, until the upstream "just" project has finished their "include" functionality.

- The ".just" suffix is the official upstream suffix for modular justfile inclusions.

usr/share/ublue-os/just/custom.just

@@ -1,28 +1,3 @@
-default:
-  @just --list
-
-bios:
-  systemctl reboot --firmware-setup
-
-changelogs:
-  rpm-ostree db diff --changelogs
-
-distrobox-boxkit:
-  echo 'Creating Boxkit distrobox ...'
-  distrobox create --image ghcr.io/ublue-os/boxkit -n boxkit -Y
-
-distrobox-debian:
-  echo 'Creating Debian distrobox ...'
-  distrobox create --image quay.io/toolbx-images/debian-toolbox:unstable -n debian -Y
-
-distrobox-opensuse:
-  echo 'Creating openSUSE distrobox ...'
-  distrobox create --image quay.io/toolbx-images/opensuse-toolbox:tumbleweed -n opensuse -Y
- 
-distrobox-ubuntu:
-  echo 'Creating Ubuntu distrobox ...'
-  distrobox create --image quay.io/toolbx-images/ubuntu-toolbox:22.04 -n ubuntu -Y
-
 setup-flatpaks:
   #!/usr/bin/env bash
   echo 'Installing flatpaks from the ublue recipe ...'
@@ -60,14 +35,3 @@ setup-gaming:
 nix-me-up:
   echo 'Setting phasers to kill. Installing nix.'
   /usr/bin/ublue-nix-install
-
-update:
-  #!/usr/bin/env bash
-  echo "Staging system update..."
-  rpm-ostree update
-
-  echo "Updating Flatpaks..."
-  flatpak update -y
-
-  echo "Updating Distroboxes..."
-  distrobox upgrade -a