This commit is not precisely atomic, but the changes have evolved, summary:

* The ENV-based interface has been moved upwards, the generator has now a conventional
  initializer.

* RAILS_VERSION is now assumed to be a Git tag. A blank RAILS_VERSION means edge guides.

* In consequence, the EDGE env variable is gone.

* The "local" version is also gone, the current SHA1 is computed for edge guides.

* Assumes guides are generated from a repo checkout (time ago users could
  generate them from gems.)

* The WARNINGS flag is gone in consequence, you cannot disable warnings.

* The `api_link` Markdown helper is fixed.

* Docs about usage have one single place: rake guides:help.

* Links in guides have been revised.

Switch extension 👉 format as it's more apt here.

guide

ref #26404

[ci skip]

To make sure that the user won't look at a feature that doesn't
already exist if they are looking at a previous version of the guides,
let's automatically inject the Rails version the guides have been
generated against.

The guide contains a typo in the "local_assigns" section, where
rendering a model named 'Article' via `render @articles` is shown to
render a partial called `_articles.html.erb`, when in fact the necessary
partial name is `_article.html.erb`

Update i18n.md

Addressed Feedback

`judgment` is the preferred form as Rails usually follows en-us spelling
not en-gb.

This will help people figure out what credentials and tables are required to 
run ActiveRecord specs.

[ci skip]

Since `Thor::HELP_MAPPINGS` contains `-D`, so `bin/rails -D` show
rails's help. But, in Rails 5.0.1, `bin/rails -D` show the description
of rake task. I think that it is better to have the same behavior.

The exact inspect output of a BigDecimal is out of scope for what we're trying
to communicate about `dup` and `duplicable?` here.

Adding two examples distracts is disctracting, so keep the docs from before
since our minimal version is Ruby 2.2.2.

[ Koichi ITO, Jon Moss, Kasper Timm Hansen ]

This reverts commit 2163874d, reversing
changes made to 46fdbc52.

cf. https://github.com/ruby/bigdecimal/pull/42

The Numeric extensions like 1.day, 1.month, etc. shouldn't know
how the internals of ActiveSupport::Duration works.

    ActiveSupport::Duration.parse('P3Y') == 3.years # It should be true

Duration parsing made independent from any moment of time:
Fixed length in seconds is assigned to each duration part during parsing.

Changed duration of months and years in seconds to more accurate and logical:

 1. The value of 365.2425 days in Gregorian year is more accurate
    as it accounts for every 400th non-leap year.

 2. Month's length is bound to year's duration, which makes
    sensible comparisons like `12.months == 1.year` to be `true`
    and nonsensical ones like `30.days == 1.month` to be `false`.

Calculations on times and dates with durations shouldn't be affected as
duration's numeric value isn't used in calculations, only parts are used.

Methods on `Numeric` like `2.days` now use these predefined durations
to avoid duplicating of duration constants through the codebase and
eliminate creation of intermediate durations.

Add missing release notes entry for https://github.com/rails/rails/commit/f02a35b86efea24f1e2ab684bc8081ced5eb3b1a [ci skip]

Rails automatically finds bi-directional associations between models
with well-named associations. However, when using non-standard naming,
you have to use :inverse_of to explicitly tell Rails about the bi-
directional association.

With reference to #27516

[ci skip]

This reverts commit 6961afef.

We were not able to keep backward compatibility in this case so it is
better to ask people to upgrade with cold cache.

[ci skip]

- Replaced MD5 => SHA256 where necessary
- Updated sprockets manifest example

See: https://github.com/rails/sprockets/blob/dd9b3b53c06c602c49057bc4a426c1a446ed7a4a/lib/sprockets.rb#L26

Complex and Rational objects can not be duplicated.

Duplicable objects in Ruby 2.4.0 include:
- most numbers
- symbols
- `nil`
- `true`
- `false`

Rewrite bits and pieces to mention the command infrastructure.

* How to disable scaffolds.scss in generators.md

* Improves workflow customization text in generators.md

When I was looking for how to default the entire routes file to JSON I could only find the postfix option. It was mentioned on IRC that you could do this with a block as well and I had not found it in the documentation.