docs: update docs on ICS slashing (#1490)

* update slashing docs

* add examples of jsons for double voting

* doc: add example of misbehaviour.json (#1495)

add example of misbehaviuour.json

* Update docs/docs/features/slashing.md

Co-authored-by: Philip Offtermatt <[email protected]>

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Co-authored-by: Simon Noetzlin <[email protected]>
Co-authored-by: Philip Offtermatt <[email protected]>