* New MIT Open Access books

* Alphabetic fix

* Update a Java book URL - #2990

Changes:
- The book 'Object-Oriented vs. Functional Programming' has a .csp link
- Also a sign-up is required to read the book since it's hosted at oreilly.com
- The contributing guide is against a hosting service and required sign-up.
- That is why this PR is created.

* remove ? link
Co-authored-by: NEric Hellman <eric@hellman.net>

* added spring data reference

* satisfy linter
Co-authored-by: NEric Hellman <eric@hellman.net>

* Fixed non-working links

* Newline fix

* Fixed a link missing https://

* New Packt books

* Alphabetic order fix

* Added the rest of the free Packt ebooks

Microsoft Dynamics 365 Enterprise Edition Financial Management - Third Edition was left out because it's not really relevant.

* New section fix

* Updated the missing APL books

* Removed the trailing slash

* New Packt books

* Packtpub account mentions added

* New Python books from Packt

* Fixed alphabetical order

Parallel and Concurrent Programming in Haskell is now hosted on O'Reilly Safari.

CA-Clipper 5.2 Norton Guide is not available anymore since last May, according to Archive.org and the nameserver changes made at the time. I changed the link to point to the latest archived page which seems to have quite a good coverage.

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