Add install and uninstall completions documentation

Author: wleczny

website/docs/_advanced_install.mdx

@@ -472,6 +472,8 @@ If Scala CLI was installed via the installation script, you can uninstall it wit
 ```bash
 scala-cli uninstall
 ```
+
+If you have installed Scala CLI completions into your shell, `uninstall` command uninstalls them by running [`uninstall-completions`](/docs/commands/completions) command under the hood.
 </TabItem>
 
 <TabItem value="manual-u">
@@ -573,6 +575,8 @@ If Scala CLI was installed via the installation script, you can uninstall it wit
 ```bash
 scala-cli uninstall
 ```
+
+If you have installed Scala CLI completions into your shell, `uninstall` command uninstalls them by running [`uninstall-completions`](/docs/commands/completions) command under the hood.
 </TabItem>
 
 <TabItem value="manual-u">

website/docs/commands/completions.md

@@ -0,0 +1,20 @@
+---
+title: Completions
+sidebar_position: 19
+---
+
+The completions commands can be used to install Scala CLI completions into your shell and uninstall them.
+
+## Installing completions
+To install completions run the `install-completions` command:
+
+```bash ignore
+scala-cli install-completions
+```
+
+## Uninstalling completions
+To uninstall completions run the `uninstall-completions` command:
+
+```bash ignore
+scala-cli uninstall-completions
+```