Fix path to channel files.

`rails generate channel Chat` generates `app/javascript/channels/chat_channel.js`.
See also,
railties/lib/rails/generators/rails/app/templates/app/javascript/packs/application.js.tt,
actioncable/lib/rails/generators/channel/templates/javascript/index.js.tt
by default `application.js` imports "channels", where
`app/javascript/channels/index.js` loads all the channels within
this directory and all subdirectories.

Follow up #34709
Related to #33079

In #34709 we updated the guide, but `actioncable/README.md` is still
outdated. Instead of fixing content in the file. I suggest
not duplicate the info that is already in the guide and instead remove
the info from the file and just add a message:
"You can read more about Action Cable in the
[Action Cable Overview](https://edgeguides.rubyonrails.org/action_cable_overview.html) guide."

The same approach is being used for Action Mailbox and Action Text,
see #34812 and #34878.

Fixes #33083

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http links will be redirected to the https version, but still better to
just directly link to the https version.

[ci skip] Add missing **DO NOT READ THIS FILE ON GITHUB, GUIDES ARE PUBLISHED ON http://guides.rubyonrails.org.**

Evented Redis adapter was removed in 48766e32.

Evented Redis is removed from Rails.
See #30945

In some examples and guides we are recommending to use code like:

```ruby
verified_user = User.find_by(id: cookies.signed[:user_id])
```

My suggestion is to use instead:

```ruby
verified_user = User.find_by(id: cookies.encrypted[:user_id])
```

which invites users to prefer the "newer" encrypted cookies over the
"legacy" signed cookies.

Related #28570

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The default adapter in development & test environments is not `redis` but `async`.

- Followup of https://github.com/rails/rails/pull/26125.

Replace broadcast_to with ActionCable.server.broadcast to be inline with its partner, #stream_from

Guide generation is configured to detect a guide header by a 40-or-more-hyphen long line. The Action Cable guide was missing this line, so the page title on the guides site was only showing "Ruby on Rails Guides", and the section title in the Kindle index was "Ruby on Rails Guides". Adding the header demarcation line back in fixes both of these.

Fixes #25697

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Default worker pool size was changed from 100 to 4 at #24376

In #23935, cable file was to be provided by the javascript instead of coffeescript,
doc was also been modified to use javascript.

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Follow up to 8b69f1ee