docs: add COTI ERC-7984 as a top-level product section

[plucena-coti]

Summary

Adds COTI ERC-7984 as a new top-level product section, comparing COTI's Privacy on Demand pERC20 with FHE-based implementations of the ERC-7984 confidential-token standard.

Replaces #32, which carried the same material as a single page inside the Privacy Portal developer guide before it was restructured. This PR is that restructure as one clean commit against main.

Structure

* [COTI ERC-7984](coti-erc7984/README.md)
  * [Precision and decimals](coti-erc7984/precision-and-decimals.md)
  * [Host-chain deployment](coti-erc7984/host-chain-deployment.md)
  * [Transaction economics](coti-erc7984/transaction-economics.md)
  * [Transfer semantics](coti-erc7984/transfer-semantics.md)
  * [Concurrency](coti-erc7984/concurrency.md)
  * [Deployed contracts](coti-erc7984/deployed-contracts.md)

Listed after Privacy on Avalanche in SUMMARY.md, so the PoD / PoA pair stays together.

What the section covers

• Precision and decimals — 256-bit values vs the 64-bit ceiling that caps an 18-decimal FHE token at roughly 18.4 whole units
• Host-chain deployment — any EVM chain with a PoD Inbox; no FHE precompiles, no forked EVM, no migration
• Transaction economics — ~192-byte encrypted inputs and 2-slot balances vs ~16–20 KB ZK input proofs and offloaded ciphertext blobs, plus encrypt-and-sign vs per-transaction browser proof generation
• Transfer semantics — exact encrypted allowances with standard approve / transferFrom, and over-balance transfers that resolve to an encrypted zero with no revert or observable difference
• Concurrency — multiple in-flight requests per account, ordered by a monotonic nonce
• Deployed contracts — the six pTokens live on Avalanche Fuji and Ethereum Sepolia, plus explorer classification via ConfidentialTransfer events and ERC-7984 metadata

Navigation

• New top-level entry in SUMMARY.md; nothing added to the Privacy Portal developer guide
• Cross-link added to privateerc20.sol.md, following its ERC20 comparison table
• Sub-pages cross-link into Privacy on Demand (architecture, async operations)