tariff/octopusenergy: Document API Key mode

[duckfullstop]

This PR documents the forthcoming API Key functionality in evcc. Depends on evcc-io/evcc#11555

Needs a German translation from the English (I can just get about translating DE -> EN, but the other way makes my brain hurt!

[naltatis]

I've added German translation (for the few UK citizens, that prefer reading German docs 😅)

[duckfullstop]

Thank you! Should be good to go just as soon as the feature is 🥳

[duckfullstop]

Main branch is ready for review so this can be merged as soon as that is 🎉

[naltatis]

Converting this to draft until the related pr in core is done.

[duckfullstop]

Ready for merge alongside evcc-io/evcc#11555 pending German translation (@naltatis 🥺)

[duckfullstop]

Any chance of a merge on this before people get confused by the deprecation warning?

[naltatis]

@duckfullstop sorry for the late resonse. I've missed your last messages and just saw it while revisiting the PRs. In the meantime we've introduced template-based tariffs https://docs.evcc.io/en/docs/devices/tariffs
I guess this is the place where most of the explaination from this PR should move to. These sections are autogenerated by the main project and therefore guaranteed to be in sync with releases.

https://github.com/evcc-io/evcc/blob/master/templates/definition/tariff/octopus-productcode.yaml
https://github.com/evcc-io/evcc/blob/master/templates/definition/tariff/octopus-api.yaml

I'll remove (or at least visually deprecate) the tariff section in the evcc.yaml reference area. Can you port the changes from this PR into the main repo templates?