Feat/409 update button styles

[evecrabb]

Updating button styles to match those in #387

What's been completed?

src/components/CallToAction.astro

• added shared styles such as font-weight
• implemented css nesting to improve readability
• updated secondary styles to match Figma designs provided
• used new css vars to achieve styling

src/styles/custom.css

• added custom pinks which weren't available (it seemed more graceful to add them in line with their colour-siblings than hard code them meaninglessly)
  • these have then replaced matching hsl codes to avoid duplication
• added button specific vars (I wasn't going to but the nature of the hover styles forced my hand on this one)
• updated styles for .button components to match the supplied Figma designs

References and more info

• Ticket
• refined style definition