Sovereign Cloud MicroHack updates

[janegilring]

This pull request makes several improvements to the documentation and scripts for the Microsoft Sovereign Cloud MicroHack, focusing on clarifying prerequisites, updating example resource group names, and enhancing the RBAC role assignment script for lab participants. The most significant changes are grouped below by theme.

Documentation improvements:

• Clarified MicroHack prerequisites, added a note that organizers handle most requirements for hosted events, and reformatted the list for clarity in Readme.md.
• Updated and corrected links in the MicroHack context section for greater accuracy and consistency, and added a new introductory section. [1] [2]

Resource group naming consistency:

• Updated example resource group names in deployment scripts (deploy-arcbox.ps1, deploy-localbox.ps1) to use rg-arcbox and rg-localbox instead of the previous *-shared convention. [1] [2]

RBAC role assignment script enhancements:

• Improved the description and usage examples in 3-rbac.ps1 to clarify that the script assigns multiple roles (Deployment Validator, Security Reader, Resource Policy Contributor) and targets pre-provisioned subscriptions. [1] [2] [3]
• Enhanced subscription selection logic for better user guidance and error handling, including clearer prompts and validation.
• Refactored custom role creation logic to robustly handle existing roles, update assignable scopes, and retry retrieval across subscriptions, improving reliability in lab environments.
• Improved script output formatting for clarity and added more verbose logging.