Allow line break HTML elements in Markdown files

[twz123]

Description

The idiomatic way to add line breaks in Markdown files is to use two trailing spaces at the end of a line.

This poses some challenges:

• Hard to spot if there's no editor support for highlighting whitespace.
• Editors may remove them automatically, as whitespace at the end of a line is typically regarded as superfluous.

The <br> inline HTML element serves the same purpose, so let's make an exception for it.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings