Add contributor doc for resize and cold migrate

Resize and cold migrate can be confusing since they are
very similar operations and share mostly the same code paths
but there are some notable differences. This adds a contributor
doc, similar to the evacuate-vs-rebuild contributor doc, to
try and explain things at a high level and provide enough links
so contributors can follow along and see where things fit into
the puzzle.

A sequence diagram is sorely needed for this as well but that will
come in a separate change.

Change-Id: I11b401a3f874226fdc20a0ee0bd518192f70fa1d

doc/source/contributor/index.rst

@@ -162,6 +162,8 @@ diving in.
 
   * :doc:`/contributor/evacuate-vs-rebuild`: Describes the differences between
     the often-confused evacuate and rebuild operations.
+  * :doc:`/contributor/resize-and-cold-migrate`: Describes the differences and
+    similarities between resize and cold migrate operations.
 
 .. # NOTE(amotoki): toctree needs to be placed at the end of the secion to
    # keep the document structure in the PDF doc.
@@ -169,3 +171,4 @@ diving in.
    :hidden:
 
    evacuate-vs-rebuild
+   resize-and-cold-migrate