Move QA test for Markdown style "Common Tests - Validate Markdown Files"

[johlju]

We should move the QA test for markdown files.

https://github.com/PowerShell/DscResource.Tests/blob/dev/Meta.Tests.ps1#L861

This is text I removed from the SqlServerDsc repo that should be added (or something similar) to the guidelines on the DSC Community site.

#### Unit tests for style check of Markdown files

When sending in a Pull Request (PR) a style check will be performed on all Markdown
files, and if the tests find any error the build will fail.
See the section [Documentation with Markdown](#documentation-with-markdown) how
these errors kan be found before sending in the PR.

The Markdown tests can be run locally if the packet manager 'npm' is available.
To have npm available you need to install [node.js](https://nodejs.org/en/download/).
If 'npm' is not available, a warning text will print and the rest of the tests
will continue run.