Improve docs around Layer manager

[jps]

Description

Currently, we have several components that use the layer manager

• modal
• drawer
• select
• tooltip?
• popover?

It's not always clear to users how they should use the layer manager, we should improve each of these component pages by linking off to the layer manager and making mention the fact that if they want to adjust on what layer the component sits they will have to do it via this mechanism.

See #825 for feedback from @tbradbury

Tasks/Steps

-Each of these component documentation pages should refer to the layer – add an inline message to these pages
-Audit each of the components that make use of the layer component:
- make sure that any legacy props or references to z-index are removed if no longer in use
- mark unused z-index as deprecated

• Add extra storybook scenarios for each component to illustrate layer usage

Acceptance Criteria

As a developer, it should be straightforward how I use layer components and how I adjust.