Add 'OCaml from the very beginning' book & OCaml playground (#7083)

* Add 'OCaml from the very beginning' book

* Delete ocaml from the very beginning's PDF

* Update OCaml the very beginning book link

* Add ocaml playground

books/free-programming-books-langs.md

@@ -1597,6 +1597,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Developing Applications With Objective Caml](http://caml.inria.fr/pub/docs/oreilly-book/)
 * [Functional Programming in OCaml](https://www.cs.cornell.edu/courses/cs3110/2019sp/textbook/) - Michael R. Clarkson
 * [OCaml From the Ground Up](https://ocamlbook.org) - Daniil Baturin (HTML) (:construction: *in process*)
+* [OCaml from the Very Beginning](https://johnwhitington.net/ocamlfromtheverybeginning/) - John Whitington
 * [OCaml Scientific Computing](https://ocaml.xyz/book/) - Liang Wang, Jianxin Zhao (HTML) (:construction: *in process*)
 * [Real World OCaml](https://dev.realworldocaml.org/toc.html)
 * [Think OCaml](http://greenteapress.com/thinkocaml/index.html) - Allen B. Downey, Nicholas Monje

more/free-programming-playgrounds.md

@@ -214,6 +214,7 @@
 
 ### OCaml
 
+* [OCaml Playground](https://ocaml.org/play)
 * [Try OCaml](https://try.ocamlpro.com)