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

- [ ] 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

---

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

[docs/analytics.md]: https://github.com/cncf/techdocs/blob/77d82463750a570cdc12bfe921495ff0987f9abc/docs/analytics.md