GitHub GraphQL API doesn't work with nested collections with large depth

The query [here](https://github.com/decaporg/decap-cms/blob/86d41d79b20583041570bce30b46835a36bcecf2/packages/decap-cms-backend-github/src/GraphQLAPI.ts#L384) fails because max depth is 25 in the GraphQL API. For some reason `depth = depth * 2 + 3`, so the maximum `nested.depth` in config.yml you can use in the GraphQL API is `11` without it failing. Maybe needs a note in docs, or a safety + warning if user sets higher than 11.

I will add more detail according to the issue template if needed.