Deprecate Refinery::Plugin#version.

AFAIK Plugin#version is not used anywhere.
Remove yard rake task.
Merge pull request #2129 from refinery/remove_yard_rake_task

Remove yard rake task.
Merge remote-tracking branch 'origin/remove_plugin_version'
Improved caller logic in Refinery#deprecate functionality.
Fixed line ending.

[ci skip]
Added ability to override helper.
Use hash-sign for numbered list in "8 - Extending Models" guide
Return nil if CSS array has no contents.

Fixes #2138
Merge remote-tracking branch 'kryzhovnik/adds_ability_to_override_helpers'
Minor engine template fix.

It makes no sense to show "sorting links" if there are not many of a resource.

Fixes #2142
Mention dummy app generation which is needed to run specs.
Swap words.
use .many? instead of `.count > 1` for collection

super minor, per https://github.com/refinery/refinerycms/commit/02c74dad15a7af380037e29820e27e208dccae1f#commitcomment-2450022
(so as to be consistent. `ack \.many\?` shows plenty)
Merge pull request #2153 from harleyttd/patch-011812

use ".many?" instead of ".count > 1" for collection
Added tests for caching pages.

Fixes #2145
Update "Page Titles and URLs" guide.

Fixes #2149
Update "Using Refinery with an Existing Rails App" guide.

Fixes #2150
Update "Multiple Resources in an Extension" guide.

Fixes #2151
Fixed word transposition in guide.