Document how to enable vim_mode in /docs/vim (#14138)

## Documents:

- **Added** instructions on how to enable "Vim mode" to the
["Settings"](https://zed.dev/docs/vim#settings) of
[/docs/vim](https://zed.dev/docs/vim).

While [/docs/configuring-zed](https://zed.dev/docs/configuring-zed)
_does_ mention the `vim_mode` setting,
[/docs/vim](https://zed.dev/docs/vim) does not.

This can be confusing for users like me who went straight to the vim
doc, and could not figure out how to enable vim.

## Release Notes:

- N/A

docs/src/vim.md

@@ -241,6 +241,16 @@ As any Zed command is available, you may find that it's helpful to remember mnem
 
 ## Settings
 
+Vim mode is not enabled by default. To enable Vim mode, you need to add the following configuration to your settings file:
+
+```json
+{
+  "vim_mode": true
+}
+```
+
+Alternatively, you can enable Vim mode by running the `toggle vim mode` command from the command palette.
+
 Some vim settings are available to modify the default vim behavior:
 
 ```json