Add best practices for statvars for Custom DC (#683)

* remove references to dataclass objects

* Fix a copy-paste error.

* remove extra file

* Fix a copy-paste error.

* Remove unused file

* Update Quickstart to use explicit schema

* Update custom_dc/quickstart.md

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* Changes from Keyur

* Remove references to implicit schema in the custom data page

* Remove implicit schema from config reference

* More changes

* Update custom_dc/config.md

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* Update custom_dc/custom_data.md

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* add explanation of format option

* Start work on overhaul of custom entities page

* Finish first draft of overhaul of custom entities page

* More changes

* minor tweaks

* Update custom_dc/custom_entities.md

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* Update custom_dc/custom_entities.md

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* Update custom_dc/custom_entities.md

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* Fixes from Gemini

* Start work on extending existing variables and entities

* Add reuse/naming guidelines of statvars

* Finish draft of new statvar sections

* Minor wording fixes

* Remove entities file from this PR

* Update custom_dc/custom_data.md

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* Update custom_dc/custom_data.md

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* Corrections from Gemini and some restructuring

* Add character limit on DCIDs

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Author: kmoscoe

custom_dc/config.md

@@ -129,9 +129,7 @@ columnMappings
 
 ## groupStatVarsByProperty
 
-Optional: When set to `true`, causes the Statistical Variable Explorer to display a top-level category called "Custom Variables", and groups together variables with the same population types and measured properties. For example:
-
-![group_screenshot](/assets/images/custom_dc/customdc_screenshot10.png){: width="400"}
+Optional: When set to `true`, groups together variables in the Statistical Variable Explorer by the values of a custom property (or properties). For example, if you define a custom property for variables called `gender` which you set to `male` or `female`, setting this option will show all variables with `gender:male` together in a single group and all variables with `gender:female` together in a different group.
 
 ## Sources