docs: create additional page for cheatsheets (#4087)

* docs: create additional page for cheatsheets

* docs: update cheatsheets page and add link to html/css section

* docs: add empty line at the end of the file

free-programming-cheatsheets-en.md

+### Index
+
+* [HTML / CSS](#html--css)
+
+
+### HTML / CSS
+
+* [CSS Flexbox Cheatsheet](https://css-tricks.com/snippets/css/a-guide-to-flexbox/) - Chris Coyier on CSS-Tricks (HTML)
+