"Configuring your env" page: `rake secret` is no longer correct, needs to be `bundle exec rails secret`

It looks like the docs need a change similar to [this PR from the source code repo](https://github.com/mastodon/mastodon/pull/30917). Under [the "secrets" section of the `admin/config` page](https://docs.joinmastodon.org/admin/config/#secrets), the docs for `SECRET_KEY_BASE` say: > Generate with `rake secret`. Changing it will break all active browser sessions. According to https://github.com/mastodon/mastodon/pull/30917, the command should probably be `bundle exec rails secret`. @ciencia also called this out in [a separate issue two years ago](https://github.com/mastodon/mastodon/issues/26490#issuecomment-1677697641) and I encountered the same problem today. Using `bundle exec rails secret` worked me.