docs: fix 404s in main README.md

[ayyjohn]

Problem

some links in the product analytics section are 404ing

Changes

a number of the links in the above photo have been updated.

one note: I did my best to find what the likely replacement link should be, but there's a chance that others were intended because there isn't a 1:1 mapping of old docs -> new docs.

A lot of them seemed to just be due to product-analytics getting its own nested docs page.
It's also possible that some of these shouldn't be fixed manually but should be fixed with an automatic redirect, like how https://posthog.com/docs/data/persons is redirected from https://posthog.com/manual/persons

Does this work well for both Cloud and self-hosted?

it doesn't have an impact

How did you test this code?

pushed the code to my personal fork, clicked the links in the readme to check that they work now

[joethreepwood]

Thanks, @ayyjohn -- I'm tagging @andyvan-ph to review these as the head of the Docs team. Looks good to me, but he'll have the best read on whether these point to the right place.