docs: correct default categories and German collection term

[MichaelOv]

Follow-up to #830. Two corrections in the existing basics pages.

Default categories (EN + DE)

• Categories and attributes: Server has the Host name category by default, not Network device. Network device is assigned only to router, switch, wifi, and firewall.
• The English page now uses the category title Subnetworks instead of Subnets. The German page already uses Subnetze, which matches the interface.
• Add Subnetworks to the English spelling list.

German term for collections (DE)

• The German interface and navigation call class collections Kollektionen, but several German pages said Sammlungen. Align the prose, link texts, and the German spelling list with the interface term.
• Unrelated uses of Sammlung (for example user groups as "Sammlungen von Benutzern") stay unchanged.

Example collection and class names (EN + DE)

• Collections: replace the invented default list with the 13 collections that i-doit up ships. Class examples now use the shipped names (Network device, Monitor, Netzwerk, Prozess, Favoriten).
• Class list, Finder, Classes, IP address management: the All classes dropdown lists collections, not classes. Empty classes are shown by default, and Sort offers four orders.
• Create custom classes: the New class form only asks for Name and Collection. Icon and categories are set afterwards on the Details tab.
• Add Lokation to the German spelling list.

Checks

• EN and DE builds show no new warnings.
• cspell passes for all EN and DE pages.

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