[FEAT] Rework the design of the model-generation-app page for mobile

[tbouffard]

In the initial implementation, we see that

• the width of the text in the "3 columns" section is too small. Should be already managed in [FEAT] Add a new page for the 'Model Generation Application' #509
• there is too much text on the right column of the "how it works" section

Initial implementation: mobile screeshot