Add documentation for the hub for bicep with AVMs deployment

[mateo762]

This pull request includes significant changes to the Scenarios/Secure-Baseline/bicepWithAVM/01-Hub/README.md file. The changes improve the documentation by providing a more detailed overview, deployment instructions, parameters, networking configuration, firewall policy, monitoring, outputs, customization, and notes.

Here are the most important changes:

• The title was updated to clarify that the module uses Azure Verified Modules (AVM).
• The purpose of the module was slightly reworded for clarity.
• A new "Overview" section was added to describe what the Bicep template does.
• A "Resources Deployed" section was added to list all the resources that the template deploys.
• A "Deployment" section was added to provide step-by-step instructions on how to deploy the template.
• A "Parameters" section was added to list all the parameters that can be set in the main.bicep file.
• A "Networking Configuration" section was added to describe the configuration of the hub network.
• A "Firewall Policy" section was added to explain the deployment of the Azure Firewall with a basic policy.
• A "Monitoring" section