feat(cli): add shell completions (#14341)

docs/reference/cli/completion.md

@@ -0,0 +1,29 @@
+<!-- DO NOT EDIT | GENERATED CONTENT -->
+
+# completion
+
+Install or update shell completion scripts for the detected or chosen shell.
+
+## Usage
+
+```console
+coder completion [flags]
+```
+
+## Options
+
+### -s, --shell
+
+|      |                                          |
+| ---- | ---------------------------------------- |
+| Type | <code>bash\|fish\|zsh\|powershell</code> |
+
+The shell to install completion for.
+
+### -p, --print
+
+|      |                   |
+| ---- | ----------------- |
+| Type | <code>bool</code> |
+
+Print the completion script instead of installing it.