unbound: merge domain overrides into query forwarding

[fichtner]

Important notices

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

Domain overrides are just query forwards.

Describe the solution you like

Migrate domain overrides to query forwards and simplify the docs and remaining "overrides" section.

Describe alternatives you considered

N/A

Additional context

Discussed in the forum multiple times.