Update orders api to add the created_via filter and make it not read-only for POST endpoint

[albarin]

Adds documentation for the changes on the orders endpoint introduced in:
woocommerce/woocommerce#56977
woocommerce/woocommerce#56670

Testing steps

• Run the build script found in the root of this repo: ./build.sh
• Find the build directory created by the script, and the index.html file within. Load that file in your web browser.
• Click on the Orders > List all orders item in the left-side menu and make sure the created_via filter is documented.
• Click on the Orders > Order properties item in the left-side menu and make sure the created_via attribute does not have the read-only label anymore.

⚠️ We should not merge this until 9.9.0 is released ⚠️

[malinajirka]

LGTM, thanks!