Update architecture.rst

[ho-oto]

Hello!

I've recently undertaken the challenge of decrypting a Cryptomator Vault, relying exclusively on the existing documentation, which can be found here: https://github.com/ho-oto/cryptomator-extractor-rs

In the course of this endeavor, I noticed several areas where the documentation is lacking or incomplete. This PR is intended to address and amend these shortcomings:

• It's unclear that the signature for JWT should use a concatenation of two master keys.
• There are missing details in the scrypt parameters, specifically the values for p and dkLen.
• The chunk number used in AAD is incorrectly described as a 32-bit big-endian integer in the documentation, whereas it is actually a 64-bit integer.