docs(book): update Linux AppImage and Ubuntu build instructions

Signed-off-by: k4yt3x <i@k4yt3x.com>

docs/book/src/building/linux.md

@@ -40,14 +40,17 @@ The built binaries will be located in the `build` directory.
 Ubuntu users can use the `.justfile` to build the project automatically. The `ubuntu2404` and `ubuntu2204` targets are available for Ubuntu 24.04 and 22.04, respectively. `just` will automatically install the required dependencies, build the project, and package it into a `.deb` package file. It is recommended to perform the build in a container to ensure the environment's consistency and to avoid leaving extra build packages on your system.
 
 ```bash
-# just needs to be installed manually
-sudo apt-get update && sudo apt-get install just
+# The version of `just` in the Ubuntu repository is outdated
+# We need to compile and install `just` manually
+sudo apt-get update && sudo apt-get install cargo
+cargo install just
 
 # Clone the repository
 git clone --recurse-submodules https://github.com/k4yt3x/video2x.git
 cd video2x
 
 # Build the project
+# Before running the command, ensure ~/.cargo/bin is in your PATH
 just ubuntu2404
 ```
 

docs/book/src/installing/linux.md

@@ -15,10 +15,6 @@ Video2X packages are available for the Linux distros listed below. If you'd like
   - [archlinuxcn/video2x-qt6](https://github.com/archlinuxcn/repo/tree/master/archlinuxcn/video2x-qt6)
   - [archlinuxcn/video2x-qt6-git](https://github.com/archlinuxcn/repo/tree/master/archlinuxcn/video2x-qt6-git)
 
-## Ubuntu
+## Other Distros
 
-Ubuntu users can download the `.deb` packages from the [releases page](https://github.com/k4yt3x/video2x/releases/latest). Install the package with the APT package manager:
-
-```bash
-sudo apt-get install ./video2x-linux-ubuntu2404-amd64.deb
-```
+Users of other distros can download and use the AppImage from the [releases page](https://github.com/k4yt3x/video2x/releases/latest).