Documentation: Remove content that ought to be in standard

[jpmckinney]

Continuing from #45, this extension still contains content that is about core fields, which ought to be moved to the standard (see open-contracting/standard#707). Otherwise, the extension's documentation reads as if it adds a description field, which is not the case.

https://extensions.open-contracting.org/en/extensions/documentation_details/master/

This issue is to remove that content and add it to the standard's issue.

As part of this update, we can add an example (follow-up to #34).