Add product custom fields documentation

[octaedro]

Changes proposed in this Pull Request:

The WooCommerce PR woocommerce/woocommerce#48504 added a new REST API endpoint meant to get the product custom field names.

Closes woocommerce/woocommerce#46921

This PR adds the documentation necessary for that new endpoint.

How to test the changes in this Pull Request:

1. Checkout this branch and build the project

sh build.sh

2. After running the build process, you will see a build folder in your project.
3. Go to the build folder and open the file ./index.html in your browser.
4. Go to Product custom fields in the sidebar.
5. Verify that the documentation looks correct.

[mdperez86]

I left some comments that can help improve the documentation a little bit.

Thanks for your hands on this @octaedro

[octaedro]

Thank you @mdperez86 for the review. I just addressed your suggestions.
Could you take another look at this PR?

[mdperez86]

Thanks for addressing my suggestions! I left one more comment related to the parameters that the endpoint receives.

[mdperez86]

The parameters were updated correctly.
The documentation looks good.
Nice job @octaedro !