minor #1040 add swup progress bar plugin to docs (Yassine Fikri)

This PR was merged into the 2.x branch.

Discussion
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add swup progress bar plugin to docs

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| Tickets       | none
| License       | MIT

updating docs to include swup progress bar plugin as example

Commits
-------

0adf067 add swup progress bar example to docs

src/Swup/doc/index.rst

@@ -144,6 +144,7 @@ Stimulus controller:
     // assets/controllers/myswup_controller.js
 
     import { Controller } from '@hotwired/stimulus';
+    import SwupProgressPlugin from '@swup/progress-plugin';
 
     export default class extends Controller {
         connect() {
@@ -160,6 +161,7 @@ Stimulus controller:
         _onPreConnect(event) {
             // Swup has not been initialized - options can be changed
             console.log(event.detail.options); // Options that will be used to initialize Swup
+            event.detail.options.plugins.push(new SwupProgressPlugin()); // Adding the progress bar plugin
         }
 
         _onConnect(event) {