[Exemple]: Add missing README documentation to main pure package directories

[Davphla]

Description

The main pure package directories are missing README documentation.

Create README files for the following undocumented directories:

Core/Official Packages

• gno.land/p/nt - Contains core utility packages and libraries maintained by the Gno team

  • avl, cford32, combinederr, commondao, fqname, mdalert, mux, ownable, pausable, poa, seqid, testutils, treasury, uassert, ufmt, urequire, watchdog

• gno.land/p/sys - System-level packages

  • genesis, validators

Each README should describe the purpose of the package namespace and provide an overview of its contents.