Add a page about contract sharding to the Book

[novusnota]

And more info about contract interaction in general. Additionally, add relevant examples to the Cookbook. This may get split into separate issues.

See:

• https://tact-by-example.org/06-multiple-contract-instances
• https://tact-by-example.org/06-contract-deploy-another
• https://tact-by-example.org/06-communicating-subcontract
• https://tact-by-example.org/06-parent-child
• https://tact-by-example.org/06-authenticating-children
• https://tact-by-example.org/06-unbounded-arrays — related to #133, should be completed after it.
• https://tact-by-example.org/06-unbounded-maps
• https://blog.ton.org/how-to-shard-your-ton-smart-contract-and-why-studying-the-anatomy-of-tons-jettons
• https://docs.ton.org/develop/dapps/asset-processing/jettons