Review Elements icons implementation and guidance and update as needed

[amyhupe]

What

Review how we implement icons in Elements demos and within our products.

Decide on a preferred approach and update demos and guidance to make this clear.

Why

In Feb 2022 user research into component documentation, we heard users describing different approaches to icon implementation within components.

Some users were confused by the guidance on how to implement icons, and with the way the chevron icon was included in the Section heading component demo.

We should make sure we have a clear preferred approach to implementing icons to make our products more consistent.

Done when

• [ ] Approach is discussed and agreed
• [ ] Elements documentation is updated as needed

[sturobson]

There has been discussion around the implementation of SVGs/icons in the front-end openspace

Recently the outputted code for a demo moved from using an external SVG to inlining each SVG