* reorganize files

1. make it easier to find files
2. reduce scrolling to get to README on main view of repo

* remove tr.txt again

Added extendsclass, an online PHP playground

* ClojureScript addition

Added a new ClojureScript online playground

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

* Added Blockchain Programming Book

* Added the packt link

* Added a Kotlin Playground

* The Kotlin Playground

* Added a Flutter Playground

* Removed the Leadin Slash

* Added Ruby Playground

* Added a new line at the end of free programming playgrounds

* Fixed the spacing in free-programming-books-fa_IR.md

* Fixed More errors in the above file

* Errors fixed in free-programming-books-fa_IR.md

* Some more changes

* Restored the previous farsi file

* Added an Elm Playground

* Update free-programming-playgrounds.md

* Update free-programming-playgrounds.md

* Added Blockchain Programming Book

* Added the packt link

* Added a Kotlin Playground

* The Kotlin Playground

* Added a Flutter Playground

* Removed the Leadin Slash

* Added Ruby Playground

* Added a new line at the end of free programming playgrounds

* Fixed the spacing in free-programming-books-fa_IR.md

* Fixed More errors in the above file

* Errors fixed in free-programming-books-fa_IR.md

* Some more changes

* Restored the previous farsi file

* Added Kaggle Python course

* Add Python Runestone to interactive tutorials

* added author

* Added Python and Java playground

* add space back

* lower cased name

* fixed capitalization for Java and JavaScript

* Added Rextester to R Interactive Playgrounds

* swapped order

* switched order

* Added Blockchain Programming Book

* Added the packt link

* Added a Kotlin Playground

* The Kotlin Playground

* Added a Flutter Playground

* Removed the Leadin Slash

* Added Kaggle Python course

* Add Python Runestone to interactive tutorials

* added author

* Added Python and Java playground

* add space back

* lower cased name

* fixed capitalization for Java and JavaScript

* Added Blockchain Programming Book

* Added the packt link

* Added a Kotlin Playground

* The Kotlin Playground

* Add esfiddle

* Make alphabetical

* Remove / at the end of url

* Added GDScript

* Update free-programming-playgrounds.md

* Add javatpoint android free tutorial

* add python playground

* Add interactive new Android tutorial section

javatpoint android tutorial

* del prev change

* change python playground title

* Add "What I Wish I Knew When Learning Haskell"

Follow up #3173.

* Update free-programming-books-zh.md

Follow up #3330.

* added competitive programming link

Follow up #3398.

* Added Redis interactive tutorial

* updated docs location

* Update free-programming-playgrounds.md

Added python interpreter

* Update free-programming-playgrounds.md

Fixed linter error

Added an interpreter for python

* Add Playground Reference Site for Dart & Flutter

* fix format

* Fix url dartpad.dev format

* Fix lint

* add: kubernetes and docker play grounds

* fix: sorting in alphabetical order

* fix: remove trailing slash

* Update kotlin playground

The current kotlin playground is obsolete.
Updated to the new playground link.

* remove '/' to satisfy linter

Always check the Travis-CI output!
Co-authored-by: NEric Hellman <eric@hellman.net>

* Move a playground link to CSS section

FlexBox is a CSS feature.
This move is consistent with other files.

* Fix index in english free-programming-playgrounds

* Move a section according to alphabetical order

* Remove offline link pyfiddle.io

Shutdown confirmed: https://github.com/priyankcommits/pyfiddleio/issues/7

* Update free-programming-playgrounds.md

Added codesandbox.io under React (and removed the react reference from its listing on JavaScript).
Added a Java section and linked to repl.it
repl.it actually has sandboxes for most of the languages listed here, and maybe some others, but I didn't add them all in. The Swing support for Java is new, and has some bugs. They didn't respond to my report, so I think they are spread a little thin.

* add spaces to satisfy linter

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

* Update free-programming-playgrounds.md

* Update free-programming-playgrounds.md

This reverts commit 48478fe1.

add sqlite fiddle in SQL

* Added OCaml playground

* delint

* added Solidity

* added Solidity

* added Solidity

* bitcoin book added

* fixed ordering on bitcoin book entry

* added authors for bitcoin and solidity books

* added Bitcoin and cryptocurrency Princeton book

* elm interactive playground added (ellie)

* free elm books added

* elm podcast added

* corrected spacing

* reordered elm books into existing category

* reordered format and author

* removed trailing /s

* fixed alphabetical ordering

* Fix playgrounds alphabetical order

* Add Scala to playgrounds index

* Add online Swift playground

* remove broken links

* remove broken links

* remove broken links

* fix broken links

* fix broken links

* fix broken links

* fix broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* remove broken links

* replace dead link

* Update free-programming-playgrounds.md

* remove trailing slash

* Add Scastie

* Fix 'Remove trailing slash' warning

Add R section to playground and add R-Fiddle to it.