Add basic docs for sneaker

docs/Sneaker.rst

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+SneakerNet Transfers
+====================
+
+Privilege Level Required: Administrator.
+Version: 2.1.0 and above.
+Database Requirement: ``d8934c52bac5``
+
+Introduction
+------------
+
+SneakerNet transfers are asynchronous transfers that can be managed
+by the librarian. A SneakerNet transfer is a transfer of data from one
+site to another using human power; i.e. the data is moved by hand
+(for instance, on a USB stick, hard drives, etc.).
+
+A generic SneakerNet transfer occurs in the following steps:
+
+- A clone of a sub-set of data is created on an external device.
+- A manifest of the data is created.
+- The data is physically transferred to the destination site.
+- The manifest is used to ingest the data into the destination site.
+- A callback from the destination to the source occurs to confirm the
+  transfer has completed successfully.
+
+Specifically within the librarian, SneakerNet transfers have the
+following steps:
+
+- If this is the first time using SneakerNet, add a new store to the
+  librarian that represents the device you would like to use to
+  SneakerNet data. If the store already exists, make sure it is
+  enabled using the administrator endpoint ``set_store_state``.
+- Set up a ``CreateLocalClone`` background task on the source
+  librarian to create a copy of the data to be transferred.
+- Register the remote librarian with the source librarian and
+  vice-versa.
+- Use the ``get_store_manifest`` client operation to create a
+  manifest of the cloned store. There are a few helpful options
+  here: ``create_outgoing_transfers`` creates an ``OutgoingTransfer``
+  object for each file in the store to the ``destination_librarian``,
+  ``disable_store`` disables the store on the source librarian before
+  generating the manifest (to ensure no new data is added to the store
+  and to allow the device to be swapped out), and
+  ``mark_local_instances_as_unavailable`` marks all instances of
+  the file on the new store as unavailable.
+- This store manifest can then be saved to the device to be moved
+  along with the data. It is recommended that you back up (and
+  potentially version control) the manifests.
+- Move the device to the destination site.
+- Use the ``ingest_store_manifest`` client operation to ingest the
+  data into the destination librarian. At this point, the data is
+  only staged on the librarian, and is not yet available on the
+  store or to users.
+- The ``RecieveClone`` background task on the destination librarian
+  will create a ``File`` and ``Instance`` for each file in the store.
+  Afterwards, the destination librarian will use its database
+  entry for the source librarian to callback. As part of processing
+  this callback, the source librarian will mark its ``OutgoingTransfer``
+  as complete and create a ``RemoteInstance`` for each file that
+  has been successfully transferred.
+
+Below, we have a step-by-step guide to performing a SneakerNet transfer.