Write content for "Contribute" page

[gsambrotta]

in "Contribute" page we would like to explain people, coach and not, how they can contribute to OTS, all the different ways.

Is probably important to add:

• link to our forum
• link to our github
• link to our learning material on github
• link to code of conduction
• link to coaches guidelines (or little sum)
• link to how to be member
• link to /chapters to find their city and meetups near

Add section on "Volounteer jobs" where we explain the current Volounteer position we are in need. This should lead to a link to a trend on our forum.