Update visitor tracker coverage and contribution guidance

[VesaJuvonen]

Summary

• migrate all legacy visitor tracker references to https://m365-visitor-stats.azurewebsites.net/
• ensure all 32 assets/sample.json-backed sample-root READMEs end with the correctly scoped tracker
• clarify README/metadata requirements, current submission routing, and tracker path rules in contribution guidance and templates
• add community calls and demo sign-up guidance

Tracker audit

• legacy references removed: 45
• final legacy references: 0
• final current tracker references: 52 across 50 files
• metadata-backed sample roots covered: 32/32
• metadata-backed roots initially using legacy tracker: 25
• metadata-backed roots initially missing both domains: 7

Validation

• exact tracker coverage audit
• guidance and template assertion checks
• JSON parsing for all 32 sample metadata manifests
• git diff --check