Rename Push-Namespace to New-RequirementGroup

[zachChilders]

This change reflects a more intuitive naming to better describe the intent of groups of requirements.

• New verb instead of Push, because groups are not conceptually a stack.
• RequirementGroup instead of Namespace, so we do not imply reusability by itself.

Note:

• I have left the internal Namespace parameter alone, as it still makes sense to have a namespace, just not to refer to a group by one.
• I have added an Alias in case anyone is using Push-Namespace already.

Further, I updated documentation to reflect this change and illustrate how to properly Invoke a RequirementGroup. I also cleaned up a lot of markdownlint errors in Readme.

[chriskuech]

Read the README on unit testing please. The formatter outputs are automatically gathered and checked in for review (kind of like UI snapshot testing).