feat(footerExample): Adding a footer example

[louismaximepiton]

Previews:

• https://deploy-preview-922--boosted.netlify.app/docs/5.1/components/orange-footer
• https://deploy-preview-922--boosted.netlify.app/docs/5.1/examples/footers/

[julien-deramond]

This is great while waiting for a real component: it offers an alternative for the projects that can't wait.

Maybe we should try to use the same wording and examples than in https://system.design.orange.com/0c1af118d/p/20e9fb-footer/b/9571c3/i/66611056 (no Lorem Ipsum, same words at the same place, etc.); it would be easier to review in the future.

There is a conflict that must be resolved.

[julien-deramond]

9516ff5 + 4f47d8f are a proposal to improve the understanding of the extra CSS. I propose to embed it directly in the footer.css so that we have the correct rendering in the example. While reading the doc, the developers can copy it if needed. IMO, it is easier to understand. If you disagree, you can remove those 2 commits and we'll find another way to tackle this.