Add testimonial section and footer to landing page

[blakezimmerman]

Description

This PR adds the testimonial section and the footer to the landing page. It also makes some color improvements to light mode, namely the primary button color.

https://github.com/aptos-labs/developer-docs/assets/25761639/93e9e932-23d2-44de-8435-ecd7c76d5b3d

Checklist

• Do all Lints pass?
  • [] Have you ran pnpm spellcheck?
  • [] Have you ran pnpm fmt?
  • [] Have you ran pnpm lint?

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