docs(config): document XDG_DATA_HOME support on Windows and macOS

[CalvinAllen]

Summary

Document the new XDG_DATA_HOME environment variable support on Windows and macOS.

Details

• When XDG_DATA_HOME is set, dtvem uses $XDG_DATA_HOME/dtvem as the root directory
• When not set, the default ~/.dtvem behavior is unchanged (no breaking change)
• This matches the existing Linux behavior where XDG is already supported

Related

• CLI PR: feat(config): support XDG_DATA_HOME on Windows and macOS dtvem.cli#206