Added Next.js resource (#5924)

* Added Next.js resource

* Update free-programming-books-langs.md

Added another resource from free-courses-en.md

* Update free-programming-books-langs.md

* Update books/free-programming-books-langs.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: Eric Hellman <eric@hellman.net>
Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -107,6 +107,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
   * [Ionic](#ionic)
   * [jQuery](#jquery)
   * [meteor](#meteor)
+  * [Next.js](#next.js)
   * [Node.js](#nodejs)
   * [Om](#om)
   * [React](#react)
@@ -1211,6 +1212,12 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Your First Meteor Application, A Complete Beginner’s Guide to the Meteor JavaScript Framework](http://meteortips.com/book/)
 
 
+#### Next.js
+
+* [Mastering Next.js](https://masteringnextjs.com)
+* [The Next.js Handbook](https://flaviocopes.com/page/nextjs-handbook/) - Flavio Copes (HTML, PDF) *(email required for PDF)*
+
+
 #### Node.js
 
 * [An Introduction to libuv](https://nikhilm.github.io/uvbook/) - Nikhil Marathe  ([PDF](http://nikhilm.github.io/uvbook/An%20Introduction%20to%20libuv.pdf) - [ePub](http://nikhilm.github.io/uvbook/An%20Introduction%20to%20libuv.epub))