[Documentation] Update navigation and head items related to "Available permissions".
Problem to solve
The navigation and header for "Available permissions" is not clear that they are related to custom role permissions.
Discussion: gitlab-org/gitlab#502203[Create_new_custom_admin_rol.png] (comment 2251491616)
![gitlab-org/gitlab#502203[Create_new_custom_admin_rol.png] (comment 2251491616)](https://gitlab.com/gitlab-org/gitlab/-/issues/502203/designs/Create_new_custom_admin_rol.png#note_2251491616){kind=link}
Proposal
Navigation
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Permissions and roles
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Custom roles
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Custom permissions
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Page header
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Custom permissions