docs: date localization is out of, um, date 😏

This section needs revising: https://www.bridgetownrb.com/docs/internationalization#localizing-dates

We shouldn't need to pull in a gem which loads in a bunch of Rails dependencies. Let's see if we can work around that.

Also, I don't see anything about how to _use_ the `l` helper and access dates. I _think_ it's like this:

```ruby
l(some_date, format: :long)
```