magento/devdocs#: How to declare a new CLI command via __construct method

[atwixfirster]

Purpose of this pull request

This pull request (PR) extends a list of possible ways how to declare a custom CLI name.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/extension-dev-guide/cli-cmds/cli-howto.html

Links to Magento source code

• dev:source-theme:deploy

Thank you!

whatsnew
Add another way of declaring a new CLI command in the How to add CLI commands topic.

[devops-devdocs]

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

[dobooth]

running tests

[ghost]

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[hguthrie]

@dobooth The hand of fate has been dealt. You must add a section for whatsnew or change your labeling strategy.