docs: Improve "Get Started" button and landing page guide

*Description*:

- [ ] The current "Get Started" button may not clearly indicate that it links to the Quickstart guide. Consider updating the button text or design to make its purpose clearer.
- [ ] Additionally, the Quickstart guide itself is not beginner-friendly. We should create a more welcoming landing page experience for new users (e.g., an intro page that answers "What is Envoy Gateway?").