Remove custom errors page section from the guides

This pattern is too problematic and introduces a lot of edge cases:

  * On 4.2, the issue https://github.com/rails/rails/issues/15124 is
    back again.
  * needs to define each action for each http status otherwise the
    router raises ActionController::RoutingError (No route matches).
  * If the router has `match "/*username",...` and some action is
    missing, Rails will pick up the "match" and try to do its job.
  * encourages people to copy & paste programming. Not DRY.

[ci skip]
Fix ProtocolViolation/bind message supplies for polymorphic + pluck or group
Update guides with info about vendor/assets

fixes #17893

[ci skip]
Merge pull request #18000 from caike/master

Update guides with info about vendor/assets [ci skip]
Merge pull request #17994 from mfazekas/pluck-bind-values-master

Fix ProtocolViolation/bind message for polymorphic + pluck or group+calc
Improve the test case introduced by bd0d47e
Add test case for joined pluck

https://github.com/rails/rails/commit/39542fba54328ca048fb75a5d5b37f8e1d4c1f37#commitcomment-8938379
Merge pull request #17815 from yuki24/remove-custom-errors-page-section

Remove custom errors page section from the guides [ci skip]
English fix [ci skip]
Merge pull request #18007 from georgemillo/patch-1

English fix [ci skip]
Fix default queue name prefix delimiter

Now displays as underscore, and not an empty string, when converted from markdown to HTML.
Escaped the underscore per http://daringfireball.net/projects/markdown/syntax#em
Fixes rails/rails#18009
Merge pull request #18010 from audionerd/patch-1

Fix default queue name prefix delimiter