docs: add zcash protocol doc

docs/protocols/ur_registrys/draft/zcash.md

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+## Keystone Zcash UR Registries
+
+This protocol is based on the [Uniform Resources](https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-005-ur.md). It describes the data schemas (UR Registries) used in Zcash integrations.
+
+### Introduction
+
+Keystone's QR workflow involves two main steps: linking the wallet and signing data, broken down into three sub-steps:
+
+1. **Wallet Linking:** Keystone generates a QR code with public key info for the Watch-Only wallet to scan and import.
+2. **Transaction Creation:** The Watch-Only wallet creates a transaction and generates a QR code for Keystone to scan, parse, and display.
+3. **Signing Authorization:** Keystone signs the transaction, displays the result as a QR code for the Watch-Only wallet to scan and broadcast.
+
+Two UR Registries are needed for these steps, utilizing the Partially Created Zcash Transaction structure.
+
+### Zcash Accounts
+
+#### Unified Full Viewing Key (UFVK)
+
+UFVK is a standard account expression format in Zcash as per [ZIP-316](https://zips.z.cash/zip-0316). It consists of:
+
+1. Transparent
+2. Sprout
+3. Sapling
+4. Orchard
+
+This protocol focuses on the Transparent and Orchard components.
+
+#### CDDL for Zcash Accounts
+
+The specification uses CDDL and includes `crypto-hdkey` and `crypto-key-path` specs.
+
+```cddl
+zcash-accounts = {
+    master-fingerprint: uint32, ; the master fingerprint to identify the wallet
+    accounts: [+ zcash-ufvk],
+    ? origin: text, ; source of data, e.g., Keystone
+}
+
+zcash-ufvk = {
+    ? transparent: crypto-hdkey,
+    orchard: zcash-fvk,
+    ? name: text,
+}
+
+zcash-fvk = {
+    origin: crypto-key-path,
+    key-data: bytes,
+}
+```
+
+`zcash-ufvk` describes the UFVK of a Zcash account. Each seed has multiple accounts with different indexes. For index 0, `zcash-ufvk` should contain a BIP32 extended public key with path `M/44'/133'/0'` (transparent) and an Orchard FVK with path `M_orchard/32'/133'/0'` (Orchard).
+
+#### CDDL for Zcash PCZT
+
+```cddl
+zcash-pczt {
+    data: bytes, ; Zcash PCZT, signatures inserted after signing.
+}
+```