feat: Add assistant doctype ✨

[lethemanh]

Thanks for contributing to this documentation. If you added a doctype, please be sure that it is present

• in the Readme.md index page
• in the toc.yml page
• https://github.com/cozy/cozy-store/blob/master/src/locales/en.json
• https://github.com/cozy/cozy-store/blob/master/src/config/permissionsIcons.json
• https://github.com/cozy/cozy-store/tree/master/src/assets/icons/permissions
• https://github.com/cozy/cozy-client/tree/master/packages/cozy-client/src/models/doctypes/locales
• https://github.com/cozy/cozy-stack/blob/master/assets/locales/en.po
• https://github.com/cozy/cozy-stack/blob/master/assets/styles/cirrus.css