[Solution Nav] Update storybook (elastic#210123)

## Summary

This PR consolidates the three storybooks for Solution Side Nav into a
single one
* Removed the `Custom Panel` storybook, this will soon no longer be
supported
* Removed the `Groups` storybook`, the "four squares" group will soon no
longer be supported
* Deleted the `Complex Object` storybook, it is redundant
* Added the `General layout structure` based directly on the [Figma
design](https://www.figma.com/design/SDtdvdzPcaDZRRXMV02gSW/Solution-Side-Navigation-%5B9.X%5D?node-id=2199-4744&t=Ub8fRo5OWxzcT9Z2-4)

### Current side nav issues

The storybook that remains shows where style gaps exist with the Figma
design:

<img width="997" alt="current-side-nav"
src="https://github.com/user-attachments/assets/e5d9df7e-0449-4286-a8b5-ffe946d80546"
/>

### Checklist