Update core-repos.md

[jamesmontemagno]

No description provided.

[JonDouglas]

Adds a bit of value imo for discussions and issues.

[richlander]

The first table is too broad/not helpful. It's a catch-all, which doesn't really benefit anyone.

I think of the breakdown as:

• Platform
• App Stacks
• Tools

I'd put languages in with Platform. Feel free to change the names. We don't have to have a single bucket called "App Stacks" but that's the layer.

If there is stuff (like installer) that doesn't fit well anywhere, then put that in an Other bucket.

[jamesmontemagno]

@richlander thoughts here... I reorgainzed to: Platform, App Workloads, Libraries, Tools, Documentation