Drafted a document for withdrawals for capella

[rolfyone]

Hopefully this is a good start for documenting the capella upgrade and what users need to know.

I referenced ethdo, which is a tool we don't maintain, but it's also the only tool i know of at the moment that has this functionality...

The capella upgrade is not set to release on a specific date yet, but this kind of concept document will start being useful very soon.

ref. Consensys/teku#5680 and Consensys/teku#6482

I can't run python scripts on my mac, so I'm yet to see how this looks rendered... sorry in advance...

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[bgravenorst]

LGTM

[rolfyone]

I can't approve this because I opened the PR, but in principal it looks good to me, raised 1 nit.