Add detailed "HowTo Variable Components" section

[BlackFoundry]

explain with text (and images) the various steps about making and using Variable Components

[ollimeier]

Also see: #55

[ollimeier]

I created a very brief variable component introduction video (less than 2 min):
https://docs.fontra.xyz/tutorials/#variable-components-introduction

But it would be great if a more advanced designer like Gaëtan could create a more detailed 'How to'-page maybe here:
https://docs.fontra.xyz/how-tos/

Similar to:
https://docs.fontra.xyz/how-tos/draw/designing-with-variable-components