Add grain docs (#6859)

* Add grain docs

* Move grain docs

* Fix alphabetical order

* Fix alphabetical order

books/free-programming-books-langs.md

@@ -333,6 +333,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Run Down On x86 Intel Assembly Course](https://github.com/Ret2LC/x86-assembly) - Ret2LC
 * [Software optimization resources](http://www.agner.org/optimize/) - A. Fog
 * [The Art of Assembly Language (2003)](https://web.archive.org/web/20120525102637/http://maven.smith.edu/~thiebaut/ArtOfAssembly/artofasm.html) - Randall Hyde (PDF)
+* [The Grain Docs](https://grain-lang.org/docs/)
 * [WebAssembly friendly programming with C/C++](https://github.com/3dgen/cppwasm-book/tree/master/en) - Ending, Chai Shushan, Yushih (HTML, [:package: examples](https://github.com/3dgen/cppwasm-book/tree/master/examples))
 * [Wizard Code, A View on Low-Level Programming](https://web.archive.org/web/20170712195930/http://vendu.twodots.nl/files/wizardcode4.pdf) - Tuomo Tuomo Venäläinen (PDF)
 * [x86-64 Assembly Language Programming with Ubuntu](http://www.egr.unlv.edu/~ed/x86.html) - Ed Jorgensen (PDF)