[DOCS] add documentation for ComponentSystemRegistry.list

[aidinabedi]

Description

This PR adds documentation for property list in ComponentSystemRegistry

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[mvaligursky]

I'm not very keen on this, as this suggests the users can get the list, and modify it. I'd be happy with this:

 /**
     * Gets an array of all registered component systems. Returns a shallow copy of the internal
     * array, so modifications to the returned array will not affect the registry.
     *
     * @returns {object[]} A copy of the array of registered component systems.
     */
    getSystems() {
        return this._list.slice();
    }

[willeastcott]

Wouldn't something like toArray be a bit more JavaScripty?

[mvaligursky]

Sure!