Write intro/foundational topics

[michaelsena]

- Foundational Topics:
  - docs/foundational-topics/index.md
  - Networks: docs/foundational-topics/networks.md
  - Streams: docs/foundational-topics/streams.md
  - Accounts: docs/foundational-topics/accounts.md
  - Transactions: docs/foundational-topics/transactions.md
  - Nodes and Clients: docs/foundational-topics/nodes-and-clients.md
  - Consensus: docs/foundational-topics/consensus.md