docs: Fix docs about encodings for PIV signing command

[YourMJK]

• Fixed DER encoding of DigestInfo for PIV signing command
  • DER encoding was missing the mandatory NULL value (05 00) to ensure signatures are compatible with OpenSSL/BoringSSL
  • Matches this library's implementation:

    Yubico.NET.SDK/Yubico.YubiKey/src/Yubico/YubiKey/Cryptography/RsaFormat.cs

    Lines 172 to 173 in 76e5dd6

    	/// FF FF FF FF FF FF FF FF FF FF FF FF 00 30 31 30
    	/// 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20

  • Matches BoringSSL: https://github.com/google/boringssl/blob/5622da92e1e7bacb5d0785ff5650a5a23b143b84/crypto/fipsmodule/rsa/rsa.cc.inc#L444-L445
• Fixed typo in response APDU for PIV signing command

[Copilot]

Pull Request Overview

This PR updates the documentation for the PIV signing command to correct the DER encoding details and fix a typo in the APDU signature description.

• Adjusts the documented DER encoding from 49 to 51 bytes by including the mandatory NULL (05 00)
• Updates the block description and the number of 0xFF bytes for the YubiKey command
• Corrects a typo in the hex sequence of the RSA-2048 signature in the APDU documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/users-manual/application-piv/commands.md	Updated DER encoding details for PIV signing, including mandatory NULL byte
docs/users-manual/application-piv/apdu/auth-sign.md	Fixed typo in the hexadecimal representation for the RSA-2048 signature

[DennisDyallo]

LGTM

@dainnilsson