docs: add zkApp signing RFC page

[dannywillems]

Summary

• Add initial RFC page for documenting how zkApp transactions are signed in the
  Mina Protocol
• Add page to sidebar under Researchers > Cryptography section

This page will serve as the specification for external implementers (e.g.,
hardware wallet teams like Ledger/Zondax).

This will also be important to verify that we are actually signing zkApp correctly in the Rust codebase.

Related issues

• Start Add documentation on zkApp transaction signing #1748 (zkApp signing documentation)
• Related: Document Transaction SNARK checks as specification reference #1756 (Transaction SNARK specification)
• Related: Document Blockchain SNARK checks as specification reference #1757 (Blockchain SNARK specification)
• Related: Extract transaction types into a crate #1665 (Transaction types crate)
• Related: [Ledger App] Provide test vectors for signing zkApp #1409 (Ledger App test vectors)
• Related: Mina_base: provide test vectors for signing zkApp MinaProtocol/mina#18203 (OCaml test vectors)

Test plan

• Verify the page renders correctly on the documentation website
• Verify sidebar navigation works

[github-actions]

OCaml Reference Validation Results

Repository: https://github.com/MinaProtocol/mina.git
Branch: compatible
Status: ✓ Validation passed

Click to see full validation output

Checking OCaml references against https://github.com/MinaProtocol/mina.git (branch: compatible)
Fetching current commit from compatible...
Current OCaml commit: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c

Validating references...
========================
✓ VALID: ledger/src/account/account.rs -> src/lib/mina_base/account.ml L:201-224
  ⚠ STALE COMMIT: fc6be4c58091c761f827c858229c2edf9519e941 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/for_tests.rs -> src/lib/transaction_logic/mina_transaction_logic.ml L:2285-2285
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/for_tests.rs -> src/lib/transaction_logic/mina_transaction_logic.ml L:2351-2356
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/for_tests.rs -> src/lib/transaction_logic/mina_transaction_logic.ml L:2407
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/transaction_status.ml L:9-51
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/transaction_status.ml L:452-454
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/with_status.ml L:6-10
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/fee_transfer.ml L:76-80
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/fee_transfer.ml L:68-69
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/mina_base/coinbase.ml L:17-21
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/mod.rs -> src/lib/transaction/transaction.ml L:8-11
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/signed_command.rs -> src/lib/mina_base/signed_command_payload.ml L:34-48
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/signed_command.rs -> src/lib/mina_base/stake_delegation.ml L:11-13
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/signed_command.rs -> src/lib/mina_base/signed_command_payload.ml L:179-181
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/signed_command.rs -> src/lib/mina_base/signed_command_payload.ml L:239-243
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)
✓ VALID: ledger/src/scan_state/transaction_logic/signed_command.rs -> src/lib/mina_base/signed_command_payload.ml L:352-362
  ⚠ STALE COMMIT: 5da42ccd72e791f164d4d200cf1ce300262873b3 (current: ecbf1132c1c05dddc4629dd1ecf0a31c1af6682c)

Summary
=======
Total references found: 16
Valid references: 16
Invalid references: 0
Stale commits: 16

⚠ Warning: 16 reference(s) have stale commits
Run with --update to update them automatically

[github-actions]

✓ Code Reference Verification Passed

All code references in the documentation have been verified successfully!

Total references checked: 1
Valid references: 1

The documentation is in sync with the codebase on the develop branch.