add swup progress bar plugin to docs

[yassinefikri]

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

updating docs to include swup progress bar plugin as example

[weaverryan]

Hi!

This is a nice PR, but the issue is tricky: each plugin we add increases the download size of the controller. So, even if you aren't using the progress bar plugin, it will now be downloaded by the user (with Encore, it'll be packaged into the final built files & with asset mapper, the import will be followed and downloaded).

Instead, I think we should encourage people to create a small swup controller where they configure the exact behavior they want - so enhancing the docs here https://symfony.com/bundles/ux-swup/current/index.html#extend-the-default-behavior to show hooking up the progress bar would be 🙌 .

[yassinefikri]

I agree, i also think that extending the controller will give easier access to customize the progress bar plugin options
i will update the PR as you mentioned

[weaverryan]

Awesome - thank you Yassine!