Add documentation for RedirectUrl widget

[serhiyzhovnir]

Purpose of this pull request

This pull request (PR) adds the RedirectUrl widget documentation.
The PR is an adjusted copy of #4715 PR.
I've created it on purpose to not to increase repo size too much.

Affected DevDocs pages

These pages do not exist yet, but they will be created for the RedirectUrl widget documentation and the URLs should be like the following:

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

Links to Magento source code

• https://github.com/magento/magento2/blob/2.1/lib/web/mage/redirect-url.js - redirectUrl widget source
• https://github.com/magento/magento2/blob/2.2/lib/web/mage/redirect-url.js - redirectUrl widget source
• https://github.com/magento/magento2/blob/2.3/lib/web/mage/redirect-url.js - redirectUrl widget source
• https://github.com/magento/magento2/blob/2.1/app/code/Magento/Catalog/view/frontend/templates/product/list/items.phtml#L221 - Example of usage
• https://github.com/magento/magento2/blob/2.2/app/code/Magento/Catalog/view/frontend/templates/product/list/items.phtml#L221 - Example of usage
• https://github.com/magento/magento2/blob/2.3/app/code/Magento/Catalog/view/frontend/templates/product/list/items.phtml#L220 - Example of usage

whatsnew
Added a new topic for the Redirect Widget.

[devops-devdocs]

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

[dobooth]

@serhiyzhovnir Thanks for redoing the PR!

[dobooth]

running tests

[ghost]

Hi @serhiyzhovnir, thank you for your contribution!
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[jeff-matthews]

@dobooth, please add whatsnew.