What is the modern stack for internal engineering documentation - currently we have multiple teams storing knowledge in Notion but this is scattered and inconsistent What solutions have you found effective?

We use confluence but https://docusaurus.io/ and https://www.mkdocs.org/ are pretty good especially when coupled with a ci/cd pipeline

If you're already an Atlassian shop I'd go with Confluence. Otherwise, Docusaurus looks awesome.

I recommend docusaurus for technical Documentation. I love the versioning feature so you can version the entire documentation for a platform. With a bit of automation (and process) you can scrape all the repos in your org for

/docs/*.md

to collate everything in a single place but keeping the source of truth in the repo.