config examples for ActionCable now use Rails.application.config.action_cable

update turbolinks url [ci skip]

`schema_type` returns symbol rather than string

Some existing examples used ActionCable.server.config but for
configuring allowed_request_origins that is overridden in development
mode. The correct place to set that is
Rails.application.config.action_cable which the ActionCable initializer
loads from. I thought the other two examples should be changed as well
just in case a default value that would override a configured value is
introduced for either log_tags or disable_request_forgery_protection in
the future.

Added numeric helper into `SchemaStatements` for MySQL and PostgreSQL

doc changes to clarify asset pipeline

for rails/rails#23431

       modified:   guides/source/asset_pipeline.md
* description of asset combination from apps and gems, e.g. jquery-rails
* after @vipulnsward's related change rails/rails#23479
  correction: --skip-sprockets will prevent all of these gems, not just sass-rails and uglifier

       modified:   guides/source/working_with_javascript_in_rails.md
* noted that rails.js requires the asset pipeline

[ci skip]

Hand off the interlock to the new thread in AC::Live

A return value of `schema_type` is used by:

1. primary key type: using as `symbol.inspect`
2. normal column type: using as `symbol.to_s`

It is better to return symbol.

A full write-preferring wait can lead to deadlock.

Publish guide on "Using Rails for API-only Applications"

Fixes #23322
[ci skip]

[ci skip]

With this addition, you can add a column into the table like:

```
  create_table(:numeric_types) do |t|
    t.numeric :foo, precision: 10, scale: 2, default: 2.0
  end
```

The result of the migration above is same with:

```
  create_table(:numeric_types) do |t|
    t.decimal :foo, precision: 10, scale: 2, default: 2.0
  end
```

fix typo in `assert_enqueued_jobs` example [ci skip]

Most importantly, the original request thread must yield its share lock
while waiting for the live thread to commit -- otherwise a request's
base and live threads can deadlock against each other.

MariaDB does not support JSON type

Referencing Rails.env without checking if it's defined couples
us to Railties.

Fix by avoiding the line breaks if we don't have an env check
to rely on.

Remove unused private method

Remove duplicated `require 'arel'`

Fixes #22980.

Replace old Rails greeting references

It appears first in `lib/active_record.rb`.

Revert "No need to specify Ruby patch version on Travis CI"

Revert "When generating a mailer, you must specify Mailer in the class name in"

A couple of the READMEs were still referring the old welcome page. This
is a small change that goes over it.

[ci skip]

This reverts commit 8417d967.

In 5697bdbb and af3eb596,
add mailer suffix to generated files and classes.
Therefore, no longer need to specify `Mailer` to class name. [ci skip]

Follow up of https://github.com/rails/rails/commit/c9feea6c9ab4494b0cb0b8cf4316847854f65af6