Migrate front-matter to nesting

[stp-ip]

Is this a BUG REPORT or FEATURE REQUEST?:
feature

What happened:
Currently we haven't defined the front-matter convention to use.

What you expected to happen:
Either use nested or use flat variables in front-matter.

Anything else we need to know?:
For almost all fragments we use nesting. So that seems to be the one we should go with.

Examples
Flat:

sidebar_heading = ""
sidebar_content = ""

Nested:

[sidebar]
  heading = ""
  content = ""

[stp-ip]

A few examples:

link_title = "Link Title"
logo_title = "Logo Title"
logo = "logo.svg"
logo_text = "Logo Subtext"
->
[menu]
  title = "Link Title"

[logo]
  image = "logo.svg"
  title = "Logo Title"
  subtext ="Logo Subtext"

logo = "logo.svg"
logo_width = "500px" # optional - will default to image width
logo_height = "150px" # optional - will default to image height
-->
[logo]
  image = "logo.svg"
  width ="500px"
  height = "150px"

sidebar_title = "Sidebar"
sidebar = """
So much information  
Phone numbers  
Details  
"""
-->
[sidebar]
  title = "Sidebar"
  content = """
So much information  
Phone numbers  
Details  
"""

As we are using nesting already for buttons and more complicated variables, we should default to nesting.

[stp-ip]

Still needs input from @lauragift21 and @mpourismaiel

[stp-ip]

Would be good, if we can pin down the user facing format aka content files, static file usage etc. Then we can already document and users don't have to migrate between changes. Also gives us the ability to refactor internally without affecting the user.

[mpourismaiel]

I like nested format a lot more. It's more readable and much more manageable.

[stp-ip]

Yeah I think I agree. Looks cleaner and we make everything more consistent. So if @lauragift21 agrees, that means:
nested > flat

[lauragift21]

Sure i agree with the nested convention it looks better.