Simplifying the observable "get" example

The "get" example referenced a method on "this" called "getEach",
which is not a method of Ember.Object.

Furthermore the signature used with "getEach" in the example does not
exist in the framework.  This can be confusing to new users.
Simplify rubygems version string.

Rubygems only supports letters, numbers, and period as part of the
version string. This uses the new structure implemented in
https://github.com/emberjs/ember.js/commit/3d8d23af6d9f1387ee37d9588bd567704730ee21
and simplifies the generated rubygems_version_string for non-published
versions.
[BUGFIX beta] reduceComputed handle out of range indexes.

Make `ReduceComputedProperty`'s `DependentArrayObserver` respect out of range
indexes the same was `[].splice` and `MutableArray.replace` do, namely:

- idx >= length is treated as removing no items and pushing new items and
- idx < 0 && idx < |length| is treated as a 1-based index from the right and
- idx <= |length| is treated as 0.

Big thanks to @twokul for pairing on this fix.

Fix #3586.
Clarify that you should not override Ember.View#destroyElement
[FEATURE ember-router-loading-error-substates] flaggify substates

Much of the internal implementation for the Router had to change
to allow for substates, hence my reluctance to throw feature flags
on the previous substates commit, but we do in fact want/need
feature flags, so here they are. When the feature flag is disabled,
legacy (inconsistent/kind of broken) LoadingRoute behavior is
preserved.
Merge pull request #3615 from corsen2000/computed-get-doc-fix

Simplifying / correcting the observable "get" example
Merge pull request #3617 from machty/loading-substates

[FEATURE ember-router-loading-error-substates] flaggify substates
Don't use Ember.Error for needsFinallyFix.

As mentioned in
https://github.com/emberjs/ember.js/pull/3594/files#r7088644 this does
not need to use `Ember.Error`.
Add ember-routing-loading-error-substates to FEATURES.

[ci skip]
Merge pull request #3621 from rjackson/add_loading_states_to_features

Add ember-routing-loading-error-substates to FEATURES.
Merge pull request #3610 from lukemelia/doc-tweak

Clarify that you should not override Ember.View#destroyElement
Merge pull request #3605 from rjackson/follow_rubygems_version_rules

Simplify rubygems version string.
Improves {{action}} helper docs

* Removed references to old-style view use
* Removed superfluous handlebars script tags
Merge pull request #3608 from hjdivad/fix-3586

[BUGFIX beta] reduceComputed handle out of range indexes.
[DOC] Use proper versioning string.

[ci skip]
reduce computed @self -> @this
Merge pull request #3620 from rjackson/fixup_ember_error_usage

Don't use Ember.Error for needsFinallyFix.
Merge pull request #3622 from rjackson/use_placeholder_version_string_in_yuidoc

[DOC] Use proper versioning string.
Merge pull request #3624 from emberjs/self_to_this

reduce computed @self -> @this
Fix typo

garentee -> guarantee