Display the + button on mobile browsers

Steps to reproduce:

1. Navigate to https://nightly-2021.nuxeocloud.com/nuxeo/ on a smartphone (with Chrome mobile for example)
2. Log in with the credentials webui / webui
3. Go to your personal workspace for example
4. Observe that the + button is not displayed

Currently mobile users cannot create new documents whereas the Edit button is available, so it means that modifications from mobile browsers are allowed.

Expected behavior: the + button is displayed when using mobile browsers