Document configurable global roles

[krystofkomanec]

Summary

• add a public Administration > Roles page for configurable global roles, default roles, custom roles, and permission groups
• update users, authentication, dashboard, project access, content management, and self-hosted docs to use permission-based wording instead of fixed role assumptions
• clarify that project sharing roles are separate from global roles and keep the existing project sharing options/table unchanged
• add TODO screenshot placeholders for roles list, role create/edit form, and non-deletable role state

Verification

• make html
• git diff --check
• scanned public docs for internal permission names from the source document
• scanned for outdated fixed-role wording