Deprecate Ember.K

[cibernox]

Now that the RFC to deprecate Ember.K has been merged, this is a pretty simple task to implement, and one that is perfect for a new contributor to perform since it's not technically complex and it balanced between documentation and implementation.

The tasks to perform are divided in a couple repositories:

• Read the RFC, to understand the motivation and the outcome.
• Amend the deprecations guide in the website reporsitory.
  Information of the deprecation:
  id: ember-metal.ember-k
  until: 3.0.0
  explanation:
  The entry must explain how to automatically remove usages of Ember.K in your codebase using ember-watson.
  It must also explain that there is two options on the command: --empty and --return-this
  and that while the second is 100% compatible, the first is probably what they want. Check the
  addendum on the RFC for more information.
• Create a PR in Ember.js itselt deprecating Ember.K.
  As stated in the previous point, the id of the deprecation must be ember-metal.ember-k and the
  url must point to the entry in the deprecation guide you've just created.
  If you are not familiar with how deprecations are made, this is a good example of how the id, until and url are used

If anyone wants to go and deprecate this, I can provide support or guidance on the process. Feel free to ping me in Slack on here anytime.

[givanse]

PRs up, please let me know what changes should I make.

#14751
emberjs/website#2774

[rwjblue]

I believe this is done, thanks to all those who helped out!