feat(guidelines): drop design guidelines content and replace it with a simpler page

[julien-deramond]

Closes #888

Previews:

• Main page
• New Orange Design System for web page

[julien-deramond]

@isabellechanclou @louismaximepiton Some thoughts about the cards (not to do here but with a dedicated issue to create):

• Create specific classes for the developers in order to use the 3 authorized sizes: Large (460px), Medium (300px) and Small (220px). Something like .card-sm and .card-lg (medium size being the default size like the buttons).
• Let in the documentation all the parts regarding the ways of dealing with the sizes but precise that the recommended sizes are the larger, medium and small
• Display the 3 sizes in the examples
• Only use the medium size in the examples for all the examples
• Check the rendering and compare it to the design specs. I've spotted some inconsistencies: height of the area when there is only a title for example
• Maybe use shortcodes to avoid copy/pasting in the examples page

[julien-deramond]

We should check .github/workflows.percy.yml and how it works. I can see flags: "--snapshot-files ./docs/5.1/guidelines/**/*.html,./docs/5.1/examples/cheatsheet/index.html,./docs/5.1/examples/cheatsheet-rtl/index.html". Since we won't have the components anymore in "/guidelines" and that the 2 cheatsheet examples are hidden.