[Improvement] Adding a table/bullet list of formulations to docs/Formulations

[frederikgeth]

I've recognized a pattern where people check the docs and then have no idea there are actually a couple of nonlinear formulations implemented (ACP/ACR/IVR 3-wire, IVR with neutral).

It's probably useful to make this a bit more obvious in the docs here.

[stale]

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