[Analytics] Document alternative analytics platforms available to projects, and offer guidelines

[chalin]

• Offer CNCF projects information about available analytics platforms and broad guidelines.

/cc @nate-double-u @cjyabraham

Context: https://cloud-native.slack.com/archives/G01CSL044G0/p1734390376479439

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Update: projects seem to be confusing the existing writeup as guidelines, for example see cncf/sandbox#341 (comment). Let's try to fix this.

• Clarify that docs/analytics.md is for migration from UA - Analytics docs rework: factor out UA-to-GA4 instructions, and more #312
• Consider moving docs/analytics.md to a "migration" section - Analytics docs rework: factor out UA-to-GA4 instructions, and more #312