Update the `CONTRIBUTING.md` file to explain how to debug `Azure.Template.Doc.ps1`

[GABRIELNGBTUC]

Is your feature request related to a problem? Please describe.

I would be interested to contribute in order to add support for several new bicep features (Exported variables, functions, ...). However when testing my changes, I find a lot of difficulties to debug them. This is because Azure.Template.Doc.ps1 seems to be invoked as a new process by Invoke-PsDocument which prevents the use of break points to help in development and troubleshooting issues.

Describe the solution you'd like

Expanding CONTRIBUTING.md to explain how to debug changes made to Azure.Template.Doc.ps1