Merge branch 'coffescript_docs_migration' into 'master'

Update docs for CoffeeScript -> JavaScript.

## What does this MR do?
Removes some CoffeeScript docs and adds more JS ones

## Are there points in the code the reviewer needs to double check?
No

## Why was this MR needed?
To migrate some docs

## What are the relevant issue numbers?
#20098

## Screenshots (if relevant)
None

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5433

CONTRIBUTING.md

@@ -462,7 +462,8 @@ merge request:
     - string literal quoting style **Option A**: single quoted by default
 1.  [Rails](https://github.com/bbatsov/rails-style-guide)
 1.  [Testing](doc/development/testing.md)
-1.  [CoffeeScript](https://github.com/thoughtbot/guides/tree/master/style/coffeescript)
+1.  [JavaScript (ES6)](https://github.com/airbnb/javascript)
+1.  [JavaScript (ES5)](https://github.com/airbnb/javascript/tree/master/es5)
 1.  [SCSS styleguide][scss-styleguide]
 1.  [Shell commands](doc/development/shell_commands.md) created by GitLab
     contributors to enhance security

doc/development/gotchas.md

@@ -41,10 +41,10 @@ Rubocop](https://gitlab.com/gitlab-org/gitlab-ce/blob/8-4-stable/.rubocop.yml#L9
 
 [Exception]: http://stackoverflow.com/q/10048173/223897
 
-## Don't use inline CoffeeScript in views
+## Don't use inline CoffeeScript/JavaScript in views
 
 Using the inline `:coffee` or `:coffeescript` Haml filters comes with a
-performance overhead.
+performance overhead. Using inline JavaScript is not a good way to structure your code and should be avoided.
 
 _**Note:** We've [removed these two filters](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/hamlit.rb)
 in an initializer._
@@ -52,6 +52,7 @@ in an initializer._
 ### Further reading
 
 - Pull Request: [Replace CoffeeScript block into JavaScript in Views](https://git.io/vztMu)
+- Stack Overflow: [Why you should not write inline JavaScript](http://programmers.stackexchange.com/questions/86589/why-should-i-avoid-inline-scripting)
 - Stack Overflow: [Performance implications of using :coffescript filter inside HAML templates?](http://stackoverflow.com/a/17571242/223897)
 
 ## ID-based CSS selectors need to be a bit more specific