.NET monikerization and automation

.NET docs team has already extended the ability for .NET moniker. The next step is to have .NET docs to onboard to our automation system as other languages. Use the issue as an epic to track the work.

Basic steps:
- [x] Batch update on `docs-ref-service` readmes into right monikers.
- [x] Update the docs metadata script for putting service readme to right place.
- [x] Update the toc script and split unified to latest/preview reference.
- [x] Update the `JoinTOCPlugin` [Link](https://github.com/Azure/azure-docs-sdk-dotnet/blob/test-zsc/.openpublishing.publish.config.json#L11)
- [ ] Add toc automation work for .NET docs.
- [ ] Add service level readme automation.

Notes: Work here needs to keep **compatibility** for current work.

This is the implementation detail issue to resolve https://github.com/Azure/azure-sdk/issues/1857