Add private keys documents

docs/Formats/private-keys-format.md

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+---
+title: Private Keys
+layout: default
+parent: Formats
+---
+
+## Private Keys
+
+Private keys are essential components of encryption systems, used for decryption, signature generation, and key derivation. This document provides an overview of private keys, including their structure and encoding formats.
+
+### SecretKeyFormat
+
+- `ec_algorithm`: Specifies the elliptic curve algorithm used for private key generation.
+- `pq_algorithm`: Indicates the post-quantum algorithm used for private key generation.
+- `dh_algorithm`: Specifies the Diffie-Hellman algorithm used for private key generation.
+- `kem_algorithm`: Indicates the Key Encapsulation Mechanism (KEM) algorithm used for private key generation.
+- `master_seed`: A secret value used as the master seed for deriving other private keys.
+- `encryption_metadata`: Additional metadata related to encryption, such as nonce and key derivation parameters. This field is present if the master seed is symmetrically encrypted.
+
+```
++---------------------+
+|   ec_algorithm      |
+|   pq_algorithm      |
+|   dh_algorithm      |
+|   kem_algorithm     |
+|   master_seed       |
+|   encryption_metadata   |
++---------------------+
+```
+
+[Encryption Metadata Format](/Formats/encryption-metadata-format)