Present MessageCenter as a MagicMirror module

[bwente]

Summary

• present MMM-MessageCenter as a first-class MagicMirror² module
• remove project-specific integration references from current documentation
• remove the obsolete project-named attention configuration path
• keep the integration-neutral semantic attention contract

Validation

• npm test (87 passing)
• git diff --check
• repository and wiki search contains no project-specific references