LAB: Agent Pool - Add note on selecting specific VM sizes in Azure portal

[massimobonanni]

This pull request adds an additional note to the instructions for creating an Azure virtual machine. The new note explains how to search for and select a specific VM size if it is not listed among the suggested options.
This added note helps students unfamiliar with the Azure Portal find the right VM size for installing the Azure Pipelines agent. It can also be helpful for those using the Skillable lab to find the correct size and avoid the "policy violated" error.

• Documentation improvement:
  • Added a note explaining how to search for and select a specific VM size in the Azure portal if it is not visible in the default list, including instructions for using the "see all sizes" link and search box.