Make TocHelper public

**Is your feature request related to a problem? Please describe.**
In our project we used to use [`TocHelper.LoadYamlToc`](https://github.com/dotnet/docfx/blob/main/src/Docfx.Build/TableOfContents/TocHelper.cs#L47) to do some ToC transformations, but after upgrading to `Docfx.Build` I noticed this class is now private.

**Describe the solution you'd like**
Make `TocHelper` public.
