Docs for Azure discovery with terraform

[r0mant]

#62145 implements terraform module that allows users to setup VM auto-discovery in a single Azure account.

To complement our web UI based configuration flow, we should have guides in our docs that:

• Provide an end-to-end guide on setting up auto-discovery with the terraform module. It should cover:
  • Prerequisites
  • Terraform provider setup (Azure and teleport)
  • Example on how to configure/apply the module
  • Explain how to check auto-discovery status once it's been applied
  • Explain how to change settings (e.g. tags) and re-apply
• Reference page for the module showing all user-configurable options like the afore-linked PR includes.

The web UI flow will link to this guide:

• When user selects "terraform" way, we should link to the part of the guide that explains what provider configuration the module needs
• When user selects "manual" way, link to the end-to-end guide.