[Feature Request] Enhance docfx.schema.json MarkdigExtensions

**Is your feature request related to a problem? Please describe.**

Using docfx.json with the Schema is good help for most of the available Properties and validating right setup. Until it comes to adding the MarkdigExtensions refered from the schema file here:

https://github.com/dotnet/docfx/blob/96bf33f1315521b0f48646b2c5a18262ff21d90b/schemas/docfx.schema.json#L507-L513

The Problem with that is
- its only limiting or assuming those can be any strings
- Its not suggesting any of the usable ones
- Not validating eventual spell mistakes made unpurposly done -> docs will not build correctly with that of course

**Describe the solution you'd like**

Could we get the available values of the MarkdigExtensions Array to suggest the known ones from the github markdigExtensions repo? Saying "suggesting" and not restricting because of course maybe we defined our own extensions it should use, but I think that could be reached with "allow Additional Properties" or similar? Could think of, that there might be done this by a href to a schema thats defining the available Names maybe 🤔

**Describe alternatives you've considered**

Currently if its failing to build with maybe wrong written names of those, I'll have to go to the GH Repo src and search their list of available Extension names, which is why I came to the idea, that could be a improvement for the schema.

**Additional context**
Add any other context or screenshots about the feature request here.