Add tertiary-theme to MdButton

[aurorascharff]

Describe your changes

Implement tertiary button.
Design has been approved and added to the design system Figma by our projects' designer.

Checklist before requesting a review

• I have performed a self-review and test of my code
• I have added label to the PR (major, minor or patch)
• If new component: Is story for component created in stories-folder?
• If new component: Is tsx-file import added to packages/react/index.tsx?
• If new component: Is css-file added to packages/css/index.css?

[github-actions]

Please set a versioning label of either major, minor, or patch to the pull request.

[thomaslarsson]

Approved. If possible, change the wrapper bg color in the demo. It is slightly confusing that the button looks like a "link style button" on white backgrounds right now. (Like in the attached video) This will make it more obvious that the button is white instead of transparent. (Link style)

[aurorascharff]

Approved. If possible, change the wrapper bg color in the demo. It is slightly confusing that the button looks like a "link style button" on white backgrounds right now. (Like in the attached video) This will make it more obvious that the button is white instead of transparent. (Link style)

Det er dette som har blitt bestemt av designer. Det er visst en vanlig funksjonalitet. Selvom den ser ut som en "link". Men den er altså ikke det. Edit: Her var det snakk om demoen/storybook. Edit: Snakket med Thomas og knappen skulle være gjennomsiktig, ikke hvit!

Jeg har tenkt litt på at vi ikke tillater <a / <button differansiering i button, sånn som jeg ahr lagt til i MdLink og MdTile. Altså at hvis man bruker href så blir elementet en a-tag. Men det får vi ta som neste steg etter denne er blitt en vanlig knapp.

[thomaslarsson]

Godkjent ref siste diskusjonsrunde.