Documentation doesn't cover migration between storage backends or mixing them

# User role

Evaluating user
# Trying to achieve

Attempting to gain intuitions about what JanusGraph can do, how to interact with it usefully, and the costs of managing its infrastructure all of which I'm attempting to do in order to avoid excluding it as a useful tooling choice.
# Steps to reproduce

Scan through the docs looking for sections related to storage, reconfiguring, advanced topics, limitations, partitioning data, replicating data, etc.

# Expected outcome

The idea of migrating between storage backends is touched on in numerous related places and either documented in detail or listed as unavailable as a limitation.
If supported at all, the support or lack thereof for hot migration is described.

# Actual outcome

No mention of migration between storage backends nor a wrapper backend that provides migration between underlying backends is readily found.

Review the documentation sections related to storage, reconfiguring, advanced topics, limitations, partitioning, and replication. Determine whether migration between storage backends, mixing backends, or hot migration is supported; document the behavior and limitations, or state clearly that these capabilities are unavailable.

Written by the indexing model from the issue text.