GitHub link + Comparison Table fixes

[pierreboc]

Based on berty/community#108

We need to fix few elements:

• While download link is not available, display by a GitHub link to Berty repo on homepage.
• Display somewhere else our comparison table between Berty and other apps (https://berty.tech/faq#what-are-the-advantages-of-berty-compared-to-the-other-messengers)

@cdeleeuwe can you handle it?

[cdeleeuwe]

@pierreboc sure can do.

Github link: There is already a link in the navigation dropdown and footer. Should there maybe another more obvious button in the navigation bar? Or somewhere else on the homepage?
Comparison table: Shall I create a dedicated page under for example /compare?

[jefft0]

Closing this feature request. We plan to refresh the berty.tech web site. At that time we will decide what content needs to be moved to a different web page.