Help Wanted: Recommend using `openModal` in webapp plugin development guide

Related ticket https://mattermost.atlassian.net/browse/MM-41610

This change should be placed in a new file in the webapp plugin development guide https://developers.mattermost.com/integrate/plugins/webapp

Proposed addition to Documentation

If a webapp plugin needs to open a modal for its own use, it should use the globally exposed window.openModal function. This is recommended because it allows the core webapp to properly handle the lifecycle of the component.

Here's an example of using this function:

this.props.actions.openModal({
    modalId: 'my-plugin-id_my-modal-id',
    dialogType: MyModal,
    dialogProps: {
        someComponentProp: 'some value'.
    },
});

• Replace my-plugin-id, my-modal-id, with appropriate values.
• dialogType should be a React component implemented by the plugin. This is the component that will be instantiated when openModal is called.
• The component will receive the dialogProps object as React props.

Start with the webapp plugin development guide at developers.mattermost.com/integrate/plugins/webapp and review the linked window.openModal export for context. Add a new file containing the proposed recommendation, TypeScript example, and explanations of modalId, dialogType, and dialogProps; done means the guidance is published in the guide.