[docs] Add howto create an html email with cozy-notification

[Crash--]

We've a lib called cozy-notification used by Banks to create emails and push notifications. We should add an howto use this lib and add a link to it from https://docs.cozy.io/en/cozy-stack/workers/#sendmail-worker

[ptbrowne]

Have you seen the readme https://github.com/cozy/cozy-libs/tree/master/packages/cozy-notifications ? It may not be perfect but it's a start.

[Crash--]

Yep it's a good start ;) !! I just want to add an how to that covers all this use case: send an html email from my app to my user