Create component description for component landing pages

[amyhupe]

What

Write a description of components explaining how we define them in the Elements Design System for the Components section landing pages.

Why

• Our recent research found that people have slightly different ideas about what components are
• Currently these pages contain placeholder content

Done when

• [x] Description is drafted
• [x] Reviewed by James
• [x] Reviewed by Alex
• [x] Published in Elements

[amyhupe]

https://github.com/springernature/ux-elements-docs/pull/55