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]
Add foreign_type option for polymorphic has_one and has_many.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.
Add guides for `required` model generator option.

Related to this PR #16062

[ci skip]
Merge pull request #17988 from caike/master

Add guides for `required` model generator option
Adding information about 'allow_nil' option for 'numericality' [ci skip]
Merge pull request #17987 from andreynering/ar-validation-doc

Adding information about 'allow_nil' option for 'numericality' [ci skip]
[ci skip] fix typo in MessageVerifier#verify docs
Merge pull request #17989 from y-yagi/patch-1

[ci skip] fix typo in MessageVerifier#verify docs
Merge pull request #17970 from ulissesalmeida/foreign-type-has-many-has-one

Add foreign_type option for polymorphic has_one and has_many.
Fix ProtocolViolation/bind message supplies for polymorphic + pluck or group
Refactor `quoted_date`

Move microseconds formatting to `AbstractAdapter`.
Revert "Merge pull request #17943 from jeremywadsack/doc_cache_importability"

This reverts commit b0e7278ddbe483475d7b2e161e65ae18df91b9f8, reversing
changes made to 785d04e3109f69d0b9b9f4732179592f0ef04e52.

This is not true. It is a bug and we will fix in the latest 4.0.x
release.
Fix undesirable RangeError by Type::Integer. Add Type::UnsignedInteger.
Merge pull request #17998 from kamipo/refactor_quoted_date

Refactor `quoted_date`
Merge pull request #17793 from kamipo/fix_undesirable_range_error

Fix undesirable RangeError by Type::Integer. Add Type::UnsignedInteger.
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