idl: Add `docs` field for constants

[acheroncrypto]

Problem

Constants in the IDL does not have a docs field unlike other types.

Summary of changes

• Add docs field for IdlConst
• Adjust IDL generation to include documentation comments for constants
• Add a test case

[vercel[bot]]

@acheroncrypto is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.