Publish release packages to NuGet.org

[JeremyCaney]

Currently, our packages are exclusively published to Ignia's internal NuGet feed on Azure Artifacts. This requires that adopters know the feed URL, have an Azure DevOps account, and have that account explicitly added to Ignia's internal NuGet feed's permissions. Preferably, now that our package versions are stable, we should publish release versions directly to NuGet.org.

Steps

• [x] Update global package configuration to use new(er) symbols package format (snupkg) supported by NuGet.org
• [x] Establish NuGet.org organization profile for OnTopic
• [x] Establish Azure DevOps Release Pipeline for publishing to NuGet.org)
• [x] Publish OnTopic Editor 5.0.0 release to NuGet.org
• [x] Update the NuGet badge in the README files to link to NuGet.org
• [x] Remove the warnings in the README files regarding authorization requirements

[JeremyCaney]

The NuGet organization and packages were previously released as part of the main OnTopic migration to NuGet. The code changes that are specific to the OnTopic Editor were updated as part of fa1c214.