docs: document KAT sources and hash verification process

[Anusaa35]

This PR adds a new section in the README clarifying the source of truth for KAT verification in liboqs.

It documents:

• How the KAT executables generate deterministic outputs
• That outputs are normalized and hashed using SHA256
• Where the expected hash values are stored (tests/KATs/.../kats.json)
• How these values are used during testing for verification

This improves transparency and maintainability by clearly documenting the origin and usage of KAT hash values.

[dstebila]

Hi saanvi, thanks for the contribution. I think improving documentation around the known answer tests is good. I'm not sure it needs to go in the top-level README file; perhaps a new README or info file in the tests/KATs folder? As well, it doesn't actually state the source of the KATs for each algorithm (e.g., the stateful signature algorithms were never part of the NIST PQC process).