docs: add completions docs

docs/pages/completions.mdx

@@ -0,0 +1,31 @@
+import { Tabs } from "nextra/components"
+
+# Installing completions
+
+Based on the shell you are using, you can install completions with the following commands:
+
+<Tabs items={["bash", "fish", "zsh", "nushell"]}>
+    <Tabs.Tab>
+        ```bash copy
+        rwalk --completions bash > /usr/local/etc/bash_completion.d/
+        ```
+    </Tabs.Tab>
+    <Tabs.Tab>
+        ```bash copy
+        rwalk --completions fish > ~/.config/fish/completions/rwalk.fish
+        ```
+    </Tabs.Tab>
+    <Tabs.Tab>
+        ```bash copy
+        rwalk --completions zsh > /usr/local/share/zsh/site-functions/_rwalk
+        ```
+    </Tabs.Tab>
+    <Tabs.Tab>
+        ```bash copy
+        mkdir $"($nu.default-config-dir)/completions"; rwalk --completions nushell | save $"($nu.default-config-dir)/completions/rwalk.nu" -f; echo 'source $"($nu.default-config-dir)/completions/rwalk.nu"' | save -a $nu.config-path
+        ```
+    </Tabs.Tab>
+</Tabs>
+
+After running the above command, you will need to restart your shell to use the completions.
+