Document contracts' storage models

[cthulhu-rider]

• [x] alphabet
• [x] audit
• [x] balance
• [x] container
• [x] neofs
• [x] neofsid
• [x] netmap
• [ ] nns
• [x] processing
• [x] proxy
• [x] reputation
• [x] subnet

[cthulhu-rider]

storage scheme eased after #303, I'll redo all progress and finish

[cthulhu-rider]

we have WIP related to NNS (#266, #267), so it's better to postpone scheme description for this contract