Add success notifications override per resource

[fzaninotto]

Problem

The snackbar that confirms the creation/update/deletion use a generic "element" name. It's impersonal and sometimes confusing.

Solution

Use a per-resource notification message, and fall back to the previous message.

The per-resource messages are:

• resources.[resource].notifications.created
• resources.[resource].notifications.updated
• resources.[resource].notifications.deleted

The default notification remains the same, however developers can override it using the translation messages, e.g.

const customEnglishMessages: TranslationMessages = {
    ...englishMessages,
    resources: {
        customers: {
            name: 'Customer |||| Customers',
            // ...
            notifications: {
                created: 'Customer created |||| %{smart_count} customers created',
                updated: 'Customer updated |||| %{smart_count} customers updated',
                deleted: 'Customer deleted |||| %{smart_count} customers deleted',
            },
        },
    },
};

French:

Supersedes #10201

How To Test

1. Launch the simple example
2. Edit a post, and save => see update one notification
3. Edit a post, and delete => delete one notification
4. Select several posts, and delete => delete many notification
5. Create a new post and save => create notification

To do

• Add per-resource notification message for create, update, and delete confirmation
• Fix unit tests
• Add new stories for this use case
• Add new tests for this use case
• Update the documentation

[fzaninotto]

I removed the sub sections ('navigation', 'list', etc) from the content and added this TOC instead to move all the section levels up (####=> ###). This makes the right menu much more useful.

[fzaninotto]

I rmeoved this one as we now document it in <Menu.Item>

[fzaninotto]

moved from `Buttons.md#menuitemlink

[fzaninotto]

There was a short doc for UpdateButton here, and a dedicated page in UpdateButton.md. I don't see why that button deserves a specific page. So I moved the content back here, changed the link in Reference.md to point here, and removed the UpdateButton item in the main navigation.

We must keep the UpdateButton.md file though, to avoid 404 on external links.

[fzaninotto]

As the new namespace for custom resource notifications is resources.{resource}.notifications, it made sense to use the same namespace in this demo.

[djhi]

I know this PR is already long but I feel like we're missing a lot of stories.

[djhi]

Shouldn't we either repeat the DeleteButton details for props or mention they are the same?

[djhi]

It would be nice to have more stories for at least the added props