docs: add Account Types page to Core Concepts (public, private, network)

[BrianSeong99]

Context

The Core Concepts section (recently renamed from Design) is mostly empty. Account types — public, private, and network — are fundamental concepts that are currently scattered across quick-start and PSM docs with no dedicated conceptual page.

Proposed

Add docs/core-concepts/accounts.md covering:

Content

1. Account ID structure — 120-bit encoding: type + storage mode + version
2. Public accounts — state stored on-chain, readable by anyone. Use case: faucets, public registries, DEX orderbooks
3. Private accounts — only commitment on-chain, state lives client-side. Use case: wallets, private vaults, confidential contracts
4. Network accounts — special accounts for protocol-level operations (faucets, validators). How they differ from regular public accounts.
5. Storage modes vs account types — clarify these are two orthogonal choices (AccountStorageMode vs AccountType)
6. Decision matrix — when to use which type

References

• AccountStorageMode enum in miden-base
• AccountType enum in miden-base
• AccountBuilder API in miden-client
• quick-start/accounts.md (has some coverage but tutorial-style)
• Paper: §2.1 Actors as Accounts

Approach

• Diataxis: explanation style (conceptual, not tutorial)
• Link to quick-start for hands-on setup
• Link to smart-contracts/accounts/ for SDK reference
• Cross-reference the protocol design docs for deeper technical details