docs: add marketing analytics column definitions and platform mappings

[jabahamondes]

Based on user questions about what do columns mean.

Changes

• Introduced a new document detailing column definitions for marketing analytics, including metrics and their calculations.
• Updated the marketing analytics overview to link to the new column definitions.
• Enhanced the navigation menu to include links to the marketing analytics overview and the new column definitions page.

Checklist

• [x] Words are spelled using American English
• [x] PostHog product names are in title case. It's "Product Analytics" not "Product analytics". If talking about a category of product, use sentence case e.g. "There are a lot of product analytics tools, but PostHog's Product Analytics is the best"
• [x] Titles are in sentence case
• [x] Feature names are in sentence case. It's "Click here to create a trend insight" not "... create a Trend Insight" and so on.
• [x] Use relative URLs for internal links
• [x] If I moved a page, I added a redirect in vercel.json
• [x] Remove this template if you're not going to fill it out!

https://posthog-git-feat-adding-ma-columns-docs-post-hog.vercel.app/docs/web-analytics/marketing-analytics-schema

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Project	Deployment	Review	Updated (UTC)
posthog	Ready	Preview	Dec 23, 2025 8:20pm