add new app's pages and more

• add new app's pages : matrix.md, discourse?md and mobilizon.md
• review apps.md and add "lab" section
• migration old apps'page in old-page.md
• review navigation in header and footer to better includes created pages
• add sub-menus in header
• add "se lancer" absolute button
• add blocks in _include to call recurrent infos : _include/engagements.md, _include/accompagenement-block.md and _include/others-apps-block.md
• Review se-lancer.md
• add icone-bullet in assets/css/page.sccs
• Minor page's review (accompagnement.md, liiibre.md and _include/form-se-lancer.html
• Minor spelling correction