Extensible layout
---

Expose how we render the HTML _surrounding_ rich text content as an
extensible `layouts/action_text/contents/_content.html.erb` template to
encourage user-land customizations, while retaining private API control
over how the rich text itself is rendered by moving the
`#render_action_text_content` helper invocation to the
`action_text/contents/_content.html.erb` partial.

Extensible Attachable `#to_attachable_partial_path`
---

When an application declares a canonical partial for a record, there is
no way to override which partial is used when transformed to Rich Text.
For example, a default `Person < ApplicationRecord` instance returns
`"people/person"` from calls to `#to_partial_path`, resulting in the
`app/views/people/_person.html.erb` partial being rendered.

Prior to this change, when encountering an `<action-text-attachment
sgid="...">` element, ActionText retrieved the corresponding
`Attachable` instance (usually an `ActiveRecord::Base` instance) and
transformed it to rich text HTML by rendering the partial that
corresponds to its `#to_partial_path`.

This proposed change instead invokes
`Attachable#to_attachable_partial_path`. By default,
`#to_attachable_partial_path` is an alias for `#to_partial_path`.

Guides
---

Extend the `guides/action_text_overview` document to
describe how to customize these templates, and to better illustrate how
ActionText::Attachable instances are rendered into HTML.