Create fixed links to a moved pages in PMM navigation menu

As we're moving all dashboards that currently is stored in PMM menu on the right side we need to create fixed links to a new pages instead of it, here is the list of links:

1. Title 'PMM Add Instance', URL '/graph/add-instance'
2. Title 'PMM Database Checks', URL '/graph/pmm-database-checks'
3. Title 'PMM Settings', URL '/graph/settings'
4. Title 'PMM Inventory', URL '/graph/inventory'

Please create PR to branch PMM-7618-inventory-add-instance-migration in grafana-dashboards