Revamp outdated docs

[BYK]

With the Spotlight 2.0 move (more like 4.0 but whatever 😅) we removed the overlay. This was a bit of a rugpull for existing users as we also had no clear documentation around this except for #892 and our current docs are now voefully outdated as pointed out in getsentry/sentry-javascript#17833

We gotta revamp the rest of our docs now that we have our homepage in order.

[BYK]

Mostly done with #1082 so closing as this will be ongoing work.