docs: add StrategyBase accounting doc (#1233)

**Motivation:**

This PR adds documentation about the `StrategyBase` contract and the
shares model, as well as answers to some common questions about the
accounting model.

Specifically, it covers:
* Why shares are needed for EigenLayer accounting, and rebase tokens
* The impact of direct transfers into a `StrategyBase` contract
* Inflation attacks, and an exploit scenario
* The mitigations built into the `StrategyBase` contract and any
residual risk & side effects

**Modifications:**

* Added the StrategyBaseAccounting doc
* Renamed `docs/core/accounting/SlashingEdgeCase.md` to
`docs/core/accounting/DualSlashingEdgeCase.md` (for clarity)

**Result:**

More clarity on how `StrategyBase` does shares accounting!

Author: nadir-akhtar