contributing: remove BUILDING.md references

[L-series]

There were references to BUILDING.md in files which were ported from
mlkem. Change these to point towards the nix setup information in
CONTRIBUTING.md.

[mkannwischer]

Thanks @L-series for pointing this out. Can we instead align with mlkem-native and add a BUILDING.md?
Diverging in sripts like autogen and format does not look appealing.

[L-series]

Thanks @L-series for pointing this out. Can we instead align with mlkem-native and add a BUILDING.md? Diverging in sripts like autogen and format does not look appealing.

I do think it would be good to add BUILDING.md to the project, and I can make this change shortly. However, I've noticed that even in mlkem, the BUILDING.md file does not actually contain the explicit information on how to install/build with nix, it simply points the user to CONTRIBUTING.md in the context of running CMBC and HOL proofs. E.g.:

environment; see nix/cbmc.
See CONTRIBUTING.md for instructions on how to setup and use nix.

Would it make more sense to change the references from BUILDING to CONTRIBUTING in autogen and format in both projetcts? Please let me know what you think.

@hanno-becker @mkannwischer