Add a "Concept" article on storage options for Spin

[macolso]

Key value stores, redis databases, MySQL oh-my! So many places to store data for our severless functions. This article would take a high level approach to explain what the various storage options are today on Spin, when they should be used, key differences in performance/consistency guarantees, etc. and then link out to appropriate tutorials to help the reader get started.

[macolso]

#430

[quyumkehinde]

This seems to have been done - #355

[itowlson]

@quyumkehinde Reopening as that PR is a KV tutorial and does not provide coverage of different storage options or of guarantees.