Docs: Guidelines for branding

[Febakke]

Guidelines

We need to document how we intendt users to use the brand feature.

• [ ] What components are brandable
• [ ] How do you set up a colorscale
• [ ] How do you apply it to your Figma file
• [ ] How to fork and use it correctly

[mrosvik]

Ser for meg en artikkel under "Grunnleggende" som forklarer branding i designsystemet, og hvorfor noen komponenter er brandable og andre ikke. :)

[mrosvik]

This is partly done in https://github.com/digdir/designsystemet/pull/2137 , but I guess we should write more about which components is brandable under https://next.designsystemet.no/grunnleggende/for-designere/eget-temare