Update landing page design

[zanete]

Why: Sub of #878
What: Updated landing page Layout for the Watchers cohorts

Scope of work:

• replace “Impact Framework is a way to compute and report the environmental impacts of software applications accurately.” with “The solution to opaque carbon reporting”
• Make buttons more prominent and above the video
• Remove current CTAs (read the docs and explore IF plugins)
• Make CTA 1: Become a watcher - https://grnsft.org/if/become-a-watcher
• Make CTA 2: Quick start - takes to the quick start page takes to https://if.greensoftware.foundation/users/quick-start
• Include Quick start in the top nav as well

[osamajandali]

@zanete Done