Define the strategy for creating a new azd template repo in Azure-Samples

[vhvb1989]

The current approach is to create a private repository with a staging branch and to make the repository public once it is ready.
For the time it becomes public, we need to create a main branch as the default and use the commits from staging. Then staging should continue to be the development vehicle for that template

New suggested approach

• Create public branch since the beginning.
• Create main branch as default with placeholder files (like a work-in-progress Readme file and files)
• Create and start using the staging branch for building the template
• Once the template is ready, merge staging to main to make the template public (optionally creating a release tag to add versioning to the template)

[danieljurek]

Repos with nothing in the main branch look pretty bare. Is there a way to programmatically construct something to put in README... otherwise people will stumble across these repos and wonder why they're empty and when they'll be filled.

On merge to main:

• Update the repo description using the first paragraph from README.md
• Set the topics
• Ensure repo is not private? (will we ever have a need to release to main and keep the repo private?)