Documentation for staged writes

[poodlewars]

We have https://docs.arcticdb.io/tutorials/parallel_writes/ but it doesn't really explain the intention of staged writes (to fold the chunk together in to one symbol upon finalize, without losing any of them). The page should also describe the other methods that support staging.

There are explanations in our docstrings, but it would be good to have a proper tutorial about staging in the website as it is a complex topic.

We should also make sure the methods are in the API docs, and revamp their docstrings if needed.