Removed the link to O'Reilly Commons because it doesn't exist anymore.

backbone js book was duplicated in js and backbonejs section so I removed it from js section (#3460)

* Added Snap! Reference Manual

* Fix link

* Fixed build error

Ruby User's Guide was pointing to a "Page not found" location (as described in issue #3448 ) but ruby-doc.org hosts almost 1:1 version of that same document so I updated the link to point there.

* Added "High Performance Accelerated Websites" book

* Updated order

* Added MOOC: openSAP

* Update free-programming-books.md

Added the book "SAP Code Style Guides - Clean ABAP", which is an adoption of Robert C. Martin's "Clean Code" for the language ABAP. The license is CC BY 3.0.

* Add Software Architecture Reactive Microsystems: The Evolution of Microservices

* fixed linter

* Adding Think Julia, Modeling with Data, Build your own lisp

* Changing order of entries.

* Added elements of programming to C++ list

* Fix the alphabetical order of Elements of Programming on C++ list

* Removing a trailing slash from Elements of Programming URI

* Add Nim Basics Free Ebook

* add spaces to satisfy linter

The "official" Dive Into Python links no longer go to the books. There is no great authoritative link to use since Mark Pilgrim left the internet, but these are returned high in Google results and appear to load really quickly.

* feat: Add "How to collaborate on Github"

* fix: wrong alpha order

current ng-newsletter link results in a 500 page. found an alternate link, but I suspect it is temporary until the newsletter fixes their 500, so we may not want to update the link. either way, this PR addresses #3400

* Remove dead link from free-programming-books-tr.md

* Change Django heading to same size as other headings

* Update link for Learning Deep Architectures

* revert django heading

* undo space add

* Adding OpenMP 5.0

* Adding PDF indicator

✔ Updated link of A Scala tutorial for Java programmers

✔ Updated link of Scala by Example

* Changed to new I Love Ruby website

* satisfy linter

* Added the React.js Succinctly.

* Update free-programming-books.md

* Included the Flutter, Azure Functions and Elixir Succinctly.

* Added two more books under flutter.

* Add Book Professional Development GitLab Handbook

* moved to Open Source Ecosystem and fixed warning

* Update free-programming-books.md

Add gidti eBook

* Add author name

* Add Mathematics Seven Sketches in Compositionalit

* Update free-programming-books.md

* Add Cloud Design Patterns

* fixed blanks