Minimal design tweaks

[pedromvpg]

• CTAs more prominent
• Make navigation flat
• Add video to homepage

[m52go]

@pedromvpg loving the new aesthetic!

Feedback on the current front page call-to-action buttons motivated #355. This PR helps address that issue by making the download dropdown more prominent, but I wonder if we can do even better.

1. What do you think about making the dropdown into a button? Simpler, clearer call to action. Could mitigate lack of options (or wrong OS guess) with a simple link to downloads page.

Idea (if better-suited for a different PR, just let me know):

2. Personally the grey get-started button looks discouraging to me, or at least not encouraging.

What do you think about a different color?

3. The grey buttons look particularly bland in the mobile view...I think at least the Downloads button should be green, just like in desktop view, and the others ones possibly adjusted too.

[pedromvpg]