MRG: Updated content for the ConfigurableProduct module

[jeff-matthews]

Purpose of this pull request

This pull request (PR) adds content from the recently updated ConfigurableProduct module README.

We typically do not allow manually editing the Module Reference Guide (MRG) data file, however, I'm making an exception in this case for the following reasons:

• The updated README exists in the 2.4-develop branch of the CE repo (private), so when we run automation to update the MRG for the next 2.4.x release, the content will already be there.
• There was no where else in devdocs where it was appropriate to document this info.
• This content resolves two Magento Support tickets and could be helpful to others. Waiting until the next release, which isn't until Q3 2021 (2.4.3), is arbitrarily restrictive.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.4/mrg/ce/ConfigurableProduct.html

Links to Magento source code

• (private repo) https://github.com/magento-commerce/magento2ce/blob/2.4-develop/app/code/Magento/ConfigurableProduct/README.md

whatsnew
Added information about configuring variables through the theme view.xml file with the ConfigurableProduct module.

[jeff-matthews]

running tests

[ghost]

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