https://linear.app/unleash/issue/2-1130/documentation-about-multiple-project-roles
- Adds a section for the feature in the "Role-based Access control"
reference doc;
- Removes the mention that "Groups that *do* have a root role can't be
assigned to a project." which is no longer true;
- Adds a reference to multiple project roles in the "How to create and
assign custom project roles" guide;
- Fixes a wrong sentence in the "Assigning custom project roles" section
of the aforementioned guide;
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Co-authored-by: Thomas Heartman <thomas@getunleash.ai>