Update layout docs

[Nico Ismaili]

As a flamingo newbie I find the documentation on how to use layouts with templates to be very sparse and confusing.

Things that I am missing:

• What exactly is a layout? - Single sentence description of the feature
• Filetype of a layout

If you want to define a site template, just call different sub templates inside like

• inside of what?
• One continuous example instead of multiple small ones
• Filenames for the examples to determine the location of the example
• The difference between a "snippet" and a layout
• Full file examples at the end of the tldr; examples
• An explanation on how objects can be passed into templates like in {{template "blocks/product.html" $p}} for example

[Nico Ismaili]

I also think that this is a good-first-issue.

[Thorsten Essig]

handled by #291