Add GitHub Actions workflow for publishing odata.org site to Azure App Service staging slot

[gathogojr]

This PR adds GitHub Actions workflow for publishing odata.org site to Azure App Service staging slot

It's triggered by either of the following events:

• A direct push to the master branch
• Merging a pull request to master

The problem of publishing the odata.org site involves the following steps:

• Build the site in a jekyll/builder container
• Log in with Azure using OpenID Connected based federated identity credentials
• Push the changes to the Azure Web App

Resources:

• https://github.com/Azure/webapps-deploy
• https://github.com/Azure/actions
• https://github.com/azure/login#github-action-for-azure-login