Analytics docs rework: factor out UA-to-GA4 instructions, and more

[chalin]

• Contributes to:
  • [Analytics] Document alternative analytics platforms available to projects, and offer guidelines #283
  • Set up Netlify build config to ensure previews are not tagged as production, etc #311
• Factors our the "UA to GA4" migration page, since it is mostly being kept for historical purposes
• Enhances link checking and build configuration so that we now have distinct production and non-production builds
• Adds Netlify config via netlify.toml (this will override the config in Netlify)
• Adds guidance for onboarding sandbox projects. More is needed, but this seems like a good start. /cc @krook

Preview: https://deploy-preview-312--cncf-techdocs.netlify.app/docs/analytics/

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[nate-double-u]

Thanks Patrice, lgtm