Store asset examples

[6r1d]

Currently, the Store asset type, which provides key-value storage for asset instances, is not documented enough.

There is one short example that explains a little part about it, but we'll be able to explain things much better if we can show how to set and get keys, at least for JS and Python APIs.

This is related to the previous code documentation issue, but it would help immensely.
The details about the Store asset were already requested in the Hyperledger Iroha community, as well. Specifically, in Discord, by alvarengacarlos#2620.

[0x009922]

Should be addressed in the examples repo:

• Add Store Asset examples iroha-2-examples#12