(footer) froggit icon and social icons

• Add social icons like the GitLab footer
• delete "suivez nous" col
• Add froggit logo (green footprint) instead of the top gitlab logo in color

Docs:

• https://docusaurus.io/fr/docs/advanced/client#theme-aliases
• https://docusaurus.io/fr/docs/swizzling
• https://docusaurus.io/fr/docs/swizzling#ejecting
• https://v1.docusaurus.io/docs/fr/custom-pages