Expand documentation on Store assets

[6r1d]

Following Savu's (#448) question, there's more interest from Matias Salimbene.

Let's document the Store asset and fill in related topics by adding links to the explanations.

• Store asset access aspects: who sees the keys and values, and who could limit data visibility? According to @Mingela, this is defined by the executor and one should have a look at a list of the default permissions here https://github.com/hyperledger/iroha/blob/7523a69475a42ac4b2c0b1893b8bbd01c85331f4/smart_contract/executor/src/default/tokens.rs#L66
• Make sure there's an explanation that Store asset works as a Dictionary with keys and values?
• Make sure we point out the differences in processing with quantity-like types, as quantity-related types are "registered" and "minted"
• Explain that the Store asset is the only way to store data in the object's metadata due to this phrase in asset documentation: "There is also the Store asset type, which is used for storing key-values in object's metadata"; explain what is meant by an object. Explain what is meant by an object. And no, the other quote in metadata needs to fully explain that (The Store asset type is used for working with metadata.).
• Make sure the similarity between Store assets and metadata is pointed out, as @0x009922 noted that Store actually uses metadata during our documentation call
• Explain that a Store asset is editable and show it in detail (SetKeyValue, RemoveKeyValue), with (probably?) no limitations for the owner by default
• Add the Store asset examples using iroha_client_cli, making sure to use several types from iroha_data_model as values
• Add the Store asset examples (Store asset examples #254) on Rust, making sure to use several types from iroha_data_model as values, add keys with values, retrieve values, delete keys, transferring keys between different assets
• Add the Store asset examples for WASM, with details same as above
• Demonstrate what types can be used as values and where to look for their definitions

This list may not cover some important aspects of Store asset type behaviour.
Please feel free to comment so it is expanded and our documentation provides enough material to work with it.

[dmitrivenger]

Will take a look at this issue during our next Doc call