docs: align NUT config docs with runtime behavior

[patrix87]

This PR aligns documentation with current implementation.

• Remove obsolete nut.hostname entry
• Document nut.ups as ups@hostname
• Update quickstart config example accordingly
• Remove unused configurations

The error was introduced in this commit where hostname was removed but documentation was added mentionning it.

[Copilot]

Pull request overview

Updates NUT configuration documentation to match the current runtime configuration model (single nut.ups string rather than separate ups + hostname fields).

Changes:

• Remove the obsolete nut.hostname entry from docs.
• Document nut.ups as <ups-name>@<hostname> and update the quickstart YAML example accordingly.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/quickstart.md	Updates the minimal config example to use ups@hostname and removes nut.hostname.
docs/configuration.md	Updates the nut.ups format documentation and removes the hostname option from the config reference.

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