Add link to M@gistère

Link to M@gistère : https://magistere.education.fr/f1091

• Show link in the footer
• Show link at top of help page

The files concerning the footer are in the @theme folder. e-comBox technical documentation : https://e-combox.gitlab.io/e-combox_webapp/ ngx-admin documentation : https://akveo.github.io/ngx-admin/docs/getting-started/what-is-ngxadmin nebular documentation : https://akveo.github.io/nebular/docs/getting-started/what-is-nebular#what-is-nebular