Add mobile landing page

[smburdick]

Is your feature request related to a problem? Please describe.
Yes: #161

Describe the solution you'd like
If I point to the app in mobile, I should see "What is TQEC?" and "GitHub repo" buttons, not the circuit editor

Describe alternatives you've considered
Building a mobile phone-friendly version should wait until we have everything we want working in the browser.

Additional context
Specifically redirect smartphone users

[nelimee]

Specifically redirect smartphone users, definitely not desktop, and maybe tablets

For the moment I agree with the "nobody except computer-users".
In the long run, ergonomics could be improved to also accommodate tablet users as I can see this as a real plus.