Update Docs: Improve custom widgets documentation

[zoetrope69]

https://deploy-preview-7032--decap-www.netlify.app/docs/custom-widgets/

1. Fix the code example
   The code example for the registerEditorComponent is incorrect as the line name: 'details', should be name: 'detail',. I think this mistake has come about because we're naming the field name the same as the HTML element <details>.
   I'm proposing to rename it to contents to avoid this mistake in the future.

2. Markup headings correctly
   We're currently marking up what look like headings with a ** bold to make it look like a heading. I'm proposing we use actual headings here.

3. Remove 'results' section
   It seems to be from some previous code example where it referred to a Youtube widget. This doesn't seem to be relevant anymore.

4. Remove unnecessary links
   Looks like Gatsby create the anchor links automatically for each heading so we don't need to define them for the 'Development' and 'Production & Publishing' headings

   

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