[Bug]: Demo app is not deployed to GitHub Pages anymore

[julien-deramond]

Prerequisites

• I have searched the backlog for duplicate or closed feature requests

Describe the issue

Jekyll is used to deploy the documentation at https://orange-opensource.github.io/ods-flutter/.
It means that the content of gh-pages is not used anymore to be deployed at https://orange-opensource.github.io/ods-flutter/.
This branch can probably be deleted.

On top of that, each time there's a commit onto develop, the workflow defined at https://github.com/Orange-OpenSource/ods-flutter/blob/develop/.github/workflows/demo_app.yml fails: https://github.com/Orange-OpenSource/ods-flutter/actions/runs/8539801641/job/23395340153.

So this workflow can be deleted as well if we don't want the demo app to be available on the web.

Moreover, it means that the content of https://github.com/Orange-OpenSource/ods-flutter/blob/main/app/README.md is wrong and must be updated.

Please double-check other files as well in the repository.