Create styleguide

[ruscoder]

The language is flexible and allows getting the same result using different ways.
I'm not sure that we should enforce using the specific syntax now, but let's start from collecting best practices

Conditional logic for array

GOOD - if as array element, the undefined value will be cleared out automatically

extension:
  - "{% if ... %}":
      url: ... 

BAD - aidbox enforces extension to be a list (even on Mapping level) - although it worked in the past versions of aidbox

extension:
  "{% if ... %}":
    - url: ... 

BAD - it's generally wrong because we can not have multiple if's at object level

"{% if ... %}":
  extension:
    - url: ...