Added brief concept topics for EigenLayer core and middleware contracts (#421)

Co-authored-by: Wes Floyd <[email protected]>

Author: MadelineAu

docs/developers/Concepts/eigenlayer-contracts/_category_.json

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+{
+  "position": 1,
+  "label": "EigenLayer Contracts"
+}

docs/developers/Concepts/eigenlayer-contracts/core-contracts.md

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+---
+sidebar_position: 1
+title: EigenLayer Core Contracts
+---
+
+The EigenLayer core contracts are the set of contracts that implement the EigenLayer protocol. The EigenLayer protocol includes
+Staking, Operations, and AVS registration and allocation. The contracts for an AVS interact with the EigenLayer contracts. 
+
+We recommend new AVS developers use the [EigenLayer middleware contracts](middleware-contracts.md) as the higher level interface
+to the core contracts. 
+
+The EigenLayer core contracts are documented in the [eigenlayer-contracts](https://github.com/Layr-Labs/eigenlayer-contracts) repository. The core contracts include: 
+* StrategyManager
+* DelegationManager
+* AVSDirectory
+* Slasher
+* RewardsCoordinator.

docs/developers/Concepts/eigenlayer-contracts/middleware-contracts.md

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+---
+sidebar_position: 1
+title: EigenLayer Middleware Contracts
+---
+
+The EigenLayer middleware contracts are higher level interfaces to the [EigenLayer core contracts](core-contracts.md).
+The middleware contracts can be: 
+* Deployed as is. The exception is the ServiceManager contract used to register and deregister an AVS with EigenLayer.
+* Modified to implement logic specific to the AVS before deploying 
+* Not used. In this case, the interfaces present in the middleware contracts must be implemented in the AVS contracts.
+
+We recommend new AVS developers use the middleware contracts as the higher level interface
+to the core contracts. 
+
+The middleware contracts are documented in the [eigenlayer-middleware](https://github.com/Layr-Labs/eigenlayer-middleware) repository.
+The ServiceManagerBase contract is the reference implementation for the onchain registration and deregistration that each AVS must have.