Remove humans.txt

[coliff]

Fixes: #2553

simplify and shorten docs about humans.txt and move it to extend docs which covers other stuff not included in the project by default.
Updates tests so they pass without the humans.txt file being included.

Types of changes

• 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.