Adjusted Loader widget documentation

[serhiyzhovnir]

Purpose of this pull request

This pull request (PR) improves the documentation about Loader widget.

Improvements:

• Added examples of the widget initialization
• Added examples of adding callbacks to processStart and processStop events
• Added Code Sample and Result sections
• Also, adjusted the markup

Affected DevDocs pages

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

Links to Magento source code

• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Backend/view/adminhtml/templates/admin/page.phtml#L18
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Backend/view/adminhtml/templates/admin/page.phtml#L18
• https://github.com/magento/magento2/blob/2.2/lib/web/mage/loader.js#L14
• https://github.com/magento/magento2/blob/2.3/lib/web/mage/loader.js#L14

whatsnew
Added new code examples to the Loader widget topic.

[devops-devdocs]

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

[serhiyzhovnir]

@rogyar The requested changes were added.
Thank you!

[rogyar]

Hi @serhiyzhovnir. Please, take a look at the conflicts with the mainline.
Thank you

[serhiyzhovnir]

Hi @rogyar
The conflict is resolved.
Thank you!

[serhiyzhovnir]

Hi @jeff-matthews
The required changes were added.
Please, check the PR.
Thank you!

[jeff-matthews]

Thanks @serhiyzhovnir!

[jeff-matthews]

running tests

[jeff-matthews]

Tests failed due to markdown style rule violations. @serhiyzhovnir, please fix the following issues:

+ rake test:cicd
�[35mTesting Markdown style with mdl ...�[0m
�[31mThe Markdown linter has found 8 issues�[0m
�[33mguides/v2.2/javascript-dev-guide/widgets/widget_loader.md:9: MD009 Trailing spaces
guides/v2.2/javascript-dev-guide/widgets/widget_loader.md:50: MD009 Trailing spaces
guides/v2.2/javascript-dev-guide/widgets/widget_loader.md:58: MD009 Trailing spaces
guides/v2.3/javascript-dev-guide/widgets/widget_loader.md:9: MD009 Trailing spaces
guides/v2.3/javascript-dev-guide/widgets/widget_loader.md:50: MD009 Trailing spaces
guides/v2.3/javascript-dev-guide/widgets/widget_loader.md:58: MD009 Trailing spaces

A detailed description of the rules is available at https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

[jeff-matthews]

Markdown linting failed

[serhiyzhovnir]

Hi @jeff-matthews
We have fixed the issue with failed markdown linting.
Please, check it.
Thank you!

[jeff-matthews]

running tests

[jeff-matthews]

@serhiyzhovnir, tests failed on image link:

Running ["LinkChecker::DoubleSlashCheck", "HtmlCheck", "ImageCheck", "LinkCheck", "ScriptCheck"] on ["_site"] on *.html... 


Ran on 2803 files!


- internal image /common/images/widget/loader-widget-result.png does not exist
  *  _site/guides/v2.2/javascript-dev-guide/widgets/widget_loader.html (line 424)
  *  _site/guides/v2.3/javascript-dev-guide/widgets/widget_loader.html (line 424)
rake aborted!
HTML-Proofer found 2 failures!``` 

[jeff-matthews]

Fix broken link to image

[serhiyzhovnir]

@jeff-matthews
I've added the missed image.
Thank you!

[jeff-matthews]

running tests

[ghost]

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