Docs: Add explainer for "Network Accounts" and "Network Transactions"

"Network Accounts" and "Network Transactions" are mentioned without having proper explanation, even though they also are a vital part of Miden. Especially for developers who are trying to build on top of our infra.

I propose adding better explanation for both to the docs.

See this page (Network Transactions): https://0xmiden.github.io/miden-docs/imported/miden-base/src/transaction.html#network-transaction

See this page (Accounts): https://0xmiden.github.io/miden-docs/imported/miden-base/src/state.html