docs: fix broken icon

[dfguerrerom]

Apparently it changed? https://fontawesome.com/icons/folder-tree?s=solid

[12rambau]

only if you rely on version 6 and normally we are still supporting v5 weird

[12rambau]

ok so multiple comments here:

• the fontawesome version remain 5.15 throughout all the document we use as we are aligning on sphinx-design. I'll update the version when this MAINT: Upgrade FontAwesome dependency to latest version pydata/pydata-sphinx-theme#868 is merged
• that being said the icon is not displaying because the folder-tree is part of the paid version of fontawesome (business) so we cannot display it

I'm closing as there is no easy fix at the moment, I'll see with SEPAL managers if we can use their pro key in the readthedoc build