Tariffs: split price list into grid and feed-in via template usages

[naltatis]

pairs with evcc-io/evcc#33735

Tariff templates can now declare usages: ["grid"] or usages: ["feedin"]. The docs use this to list a template under the matching direction and to show the right tariffs: key in its yaml example. Includes the regenerated nightly templates so the change is visible right away.

• Tariffs page: "Electricity prices" becomes two lists, "Grid tariffs" and "Feed-in tariffs". Templates without usages appear in both.
• Feed-in only templates (BKW) now show tariffs: feedin: instead of tariffs: grid: in the code example.
• Content schema accepts the new usages key, so the nightly template sync keeps building once evcc ships it.

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