[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 https://github.com/cncf/sandbox/issues/341#issuecomment-2805969237. Let's try to fix this.

• [ ] Clarify that docs/analytics.md is for migration from UA - #312
• [ ] Consider moving docs/analytics.md to a "migration" section - #312