Adding a code sample of how the modal widget can be initialized

[eduard13]

Purpose of this PR

This PR is an improvement of Modal widget, which provides an example of how the modal can be used and how the widget's options can be passed during initialization. Also the example's result preview is provided.
Moreover, an additional (important) widget's option was added to the documentation.

Affected URLs

• https://devdocs.magento.com/guides/v2.1/javascript-dev-guide/widgets/widget_modal.html
• https://devdocs.magento.com/guides/v2.2/javascript-dev-guide/widgets/widget_modal.html
• https://devdocs.magento.com/guides/v2.3/javascript-dev-guide/widgets/widget_modal.html

whatsnew
Added a new code sample showing how to initialize a modal widget and pass options to it.

[devops-devdocs]

An admin must run tests on this PR before it can be merged.

[rogyar]

CodeBase: https://github.com/magento/magento2/blob/2.1/app/code/Magento/Ui/view/base/web/js/modal/modal.js#L73

[eduard13]

Hi @jeff-matthews, the suggested changes were applied.
Thank you for the review.

[jeff-matthews]

@eduard13, are all three image files the same? If so, we only need one image and it should be saved to devdocs/common/images/. there's no need to replicate the same image.

[eduard13]

@jeff-matthews, yes they are. Interesting, I didn't know about that, thank you for noticing.
Please note that I moved it into the mentioned folder, optimized it and removed the other ones.
Thank you.

[jeff-matthews]

Perfect @eduard13! Thank you for taking the extra step to optimize the image as well. We haven't had good contributor guidelines around doing that in the past, but we'll create some soon. Thank you for being proactive.

[jeff-matthews]

running tests

[jeff-matthews]

Tests failed. Here's the log:

+ rake test:cicd
�[35mChecking links with htmlproofer...�[0m
Running ["LinkChecker::DoubleSlashCheck", "ImageCheck", "LinkCheck", "ScriptCheck"] on ["_site"] on *.html... 

Ran on 3824 files!

- internal image /common/images/js/widgets/modal-widget-result.png does not exist
  *  _site/guides/v2.1/javascript-dev-guide/widgets/widget_modal.html (line 2368)
  *  _site/guides/v2.2/javascript-dev-guide/widgets/widget_modal.html (line 2430)
  *  _site/guides/v2.3/javascript-dev-guide/widgets/widget_modal.html (line 2486)
rake aborted!
HTML-Proofer found 3 failures!```

[jeff-matthews]

instead of adding the image to common/images/js/widget, please add to common/images/widget.

[jeff-matthews]

running tests

[eduard13]

@jeff-matthews now it should be fine I guess.

[jeff-matthews]

Yes, I think so too. Almost finished running tests.

[jeff-matthews]

running tests

[ghost]

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