Document i18n "single_file" format & map widget format

[ewjoachim]

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Is your feature request related to a problem? Please describe. When writing one's config.yml, it can be frustrating not to be able to tell what format Netlify CMS will read/write. The only way to really know is to push a commit, wait for it to build and then make a change in draft, and look at the PR, whereas local development would allow getting the answers much faster.

This is especially true for non-obvious formats such as:

• In i18n, when choosing single_file, what format will the yaml have ? Are the untranslated fields in the default language or not in a language at all ?
• In the map widget, it's documented that the output is a geojson payload: is it the object directly in the yaml or a json dump that needs to be loaded ? Does it include the type/property/geometry keys or is it only the content of the geometry key ?

Describe the solution you'd like 2 solutions come to mind:

• When saving in test_repo mode, somehow show what would be saved (maybe just in console.log, but it's already enough)
• Or just simply document with a full example whenever what NetlifyCMS will write is not obvious (... or even when it's obvious)

Additional context N/A