chore: restructure now libraries layout

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• restructure the repository as a root now-libraries workspace
• move the existing NOW protocol sources/docs under protocols/
• add a stub policies/README.md for future UniGetUI/PEDM policy libraries
• update Rust workspace paths, .NET xtask solution paths, NuGet publish paths, and repository metadata URLs

Verification

• cargo metadata --locked --format-version 1
• cargo xtask rust fmt -v
• cargo xtask rust tests --no-run -v
• cargo xtask rust tests -v (54 passed)
• cargo xtask rust lints -v
• cargo xtask dotnet fmt -v
• cargo xtask dotnet build -v
• cargo xtask dotnet tests -v (106 passed)
• cargo xtask check locks -v
• cargo xtask check typos -v
• cargo package -p now-proto-pdu --allow-dirty --no-verify

Notes

• The actual GitHub repository rename to now-libraries is expected to be handled separately by DevOps.
• Historical changelog links intentionally still point to now-proto.