* Update .Net Url
As the previose was 404 not found
* Add crosslink ref to C# section on .Net section
* Update free-courses-en.md
* fix: adopt linter 1-0-2 rule
Thats is, 2 empty lines between headings / sections
Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>