Add ecash section to guide

[swedishfrenchpress]

Hey @GBKS @mouxdesign @danielnordh.

I am working on the ecash section for the design guide. I have some foundations laid down and would like to submit a Draft PR so I can preview it in a test environment.

I would like to keep pushing changes to the draft PR regularly as I will be adding illustrations, updating copy, moving things around etc...

I'm still rather new to this process so please let me know what are the best practices here.

Preview