books: add `Conquer the Command Line` (#5787)

* books: add 'Essentials - Conquer the Command Line

chore: typo

* chore: rename book

* CI bug

books/free-programming-books-langs.md

@@ -370,6 +370,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 * [Bash Reference Manual](http://www.gnu.org/software/bash/manual/bashref.html) (HTML)
 * [Bash tutorial](https://web.archive.org/web/20180328183806/http://gdrcorelec.ups-tlse.fr/files/bash.pdf) - Anthony Scemama (PDF)
 * [BashGuide](http://mywiki.wooledge.org/BashGuide) - Maarten Billemont (HTML) [(PDF)](http://s.ntnu.no/bashguide.pdf)
+* [Conquer the Command Line](https://magpi.raspberrypi.org/books/command-line-second-edition/pdf/download) - Richard Smedley (PDF)
 * [Conquering the Command Line](http://conqueringthecommandline.com/book/frontmatter) - Mark Bates (HTML)
 * [Getting Started with BASH](http://www.hypexr.org/bash_tutorial.php) (HTML)
 * [GNU Bash manual](https://www.gnu.org/software/bash/manual/bash.pdf) (PDF)