Orders: Add doc for order actions

[coreymckrill]

Adds documentation for the REST endpoint introduced in woocommerce/woocommerce#52050

Fixes #251

To test

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. Look for the new "Order actions" item in the left-side menu. Read the text and the code examples and make sure it all makes sense and there aren't any typos!

[albarin]

Looks good to me, thanks for this!

[coreymckrill]

(Noting that we shouldn't merge this until 9.5 is released!)