Update manage_stock property type

[albarin]

Closes woocommerce/woocommerce#59319

This PR updates the manage_stock product variation property to be string or boolean. Before, it was documented only as boolean, but it can also return have the parent value when the variation inherits stock settings from the parent product.

Testing steps

1. Run the build script found in the root of this repo: ./build.sh
2. Find the build directory created by the script, and the index.html file within. Load that file in your web browser.
3. Search for manage_stock occurrences and check that the type is "type": ["boolean", "string"] or boolean, string when the description refers to Stock management at variation level. (it should still be only boolean when the description is for the product level Stock management at product level.).

"manage_stock":
    "required": false,
    "default": false,
    "description": "Stock management at variation level.",
    "type": ["boolean", "string"]
}

4. Repeat step 3 for v1 and v2 documentation as well.