[Documentation Update]: Define Bicep UDT specs and usage (container issue)

[eriqua]

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Description

Document both specs and contribution guidelines related to User Defined Types (UDT) for Bicep resource modules

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1. Contribution guidelines - how to use AVMCommonTypes utility module in resource modules #1712
   Needs: Triage 🔍 +1
   
2. New Spec - All objects and array of objects SHOULD define UDT to better leverage automatic documentation and intellisense. #1713
   Needs: Triage 🔍 +1
   
3. New Spec - All UDT MUST be singular and non nullable #1714
   Needs: Triage 🔍 +1
   
4. Contribution guidelines - how to define UDT for objects and array of objects
5. New Spec - All top level custom types MUST be exported and have a type description #1715
   Needs: Triage 🔍 +1
   
6. New Spec - All custom types must be singular and MUST NOT be nullable #1716
   Needs: Triage 🔍 +1
   

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[AlexanderSehr]

Ref: Azure/bicep-registry-modules#3582 (comment)