feat(model): add insertOne() function to insert a single doc

[vkarpov15]

Summary

Fix #14843

insertOne() is similar to create(), but only supports creating a single document. insertOne() is useful for 2 reasons:

1. Consistency with the MongoDB CRUD API. Mongoose models currently support all of the MongoDB CRUD API methods except insertOne(), which is a bit of a gotcha.
2. create() with options has proven to be confusing for users, because create(doc, { session }) attempts to create 2 documents, the 2nd with a session property. I don't want to drop support for create(doc1, doc2) (spread args) because that would just create the opposite hard-to-diagnose issue where doc2 gets treated as options. insertOne() offers a neat alternative for users who just want to create 1 doc within a transaction without new.

Examples