chore(courses): Recover PR #5693. Create Greek courses list (#6157)

* Create free-courses-gr.md and added JavaScript course

check_urls=free-programming-books.md free-programming-books-gr.md

* Update free-courses-gr.md

* Bumps review EbookFoundation#5693

Recover deleted PR head:
git fetch upstream pull/5693/head:pr/anneiric/5693

anneiric <69729163+anneiric@users.noreply.github.com>

* fix(listings): greek language code `gr` > `el`

language code for greek is `el` not `gr`. See: EbookFoundation/free-programming-books#6070

Applies https://github.com/EbookFoundation/free-programming-books/pull/5693#issuecomment-938647297

* feat(courses): Add greek entry

Link recent created greek courses listing `books/free-courses-el.md` into `README.md`

Resolves EbookFoundation/free-programming-books#5693

* feat(courses): Add greek entry

Link `courses/free-courses-el.md` into `README.md`

Resolves EbookFoundation/free-programming-books#5693

Co-authored-by: anneiric <69729163+anneiric@users.noreply.github.com>

courses/free-courses-el.md

@@ -0,0 +1,8 @@
+### Index
+
+* [JavaScript](#javascript)
+
+
+### JavaScript
+
+* [Εισαγωγή Στον WEB Προγραμματισμό Με JavaScript](https://kassapoglou.github.io/javascript/javascript-programming.html) - Μιχάλης Κασάπογλου