[Layout] New primitives

[acstll]

Description

Layout "flex" primitives (see docs) could use a refresh. The main problem is responsiveness being directly coupled to the viewport via media queries, which results in bugs, see elastic/kibana#257075

They could also be considered a bit outdated e.g. EuiFlexGrid is utterly weird in the fact it has flex in the name but uses CSS grid.

Goal

Introduce a new set of layout primitives that are responsive by default and do not depend on the viewport size (media queries).

Proposed acceptance criteria

• New and modern layout primitives are introduced
• No media queries dependency
• Primitives are easily composable
• Naming: for how these relate to or replace the existing ones, we use common sense; avoid confusing names e.g. EuiFlexGrid
• Define a rollout process including deprecation schedules and the path with least breaking changes
• Documentation is updated
• Bonus: a few composition patterns are added to the docs

[mdefazio]

I just happen to pop into these today and saw this request.... and OMG YES!!

[acstll]

I still need to add a proper description to this PR elastic/kibana#283182 where I'm putting those ideas into practice… @mdefazio your input there would be great, I'll ping you again!