Theme handbook: Explain that the attachment pages are disabled on new installations

[carolinan]

Originally reported on https://core.trac.wordpress.org/ticket/61596

Issue Description

Since WP 6.4, attachment pages are disabled by default (see https://make.wordpress.org/core/2023/10/16/changes-to-attachment-pages/)
The documentation page (https://developer.wordpress.org/themes/basics/template-hierarchy/#attachment) does not mention this - thus resulting in some effort to find out why this is not working in new installations anymore.

URL of the Page with the Issue

https://developer.wordpress.org/themes/basics/template-hierarchy/#attachment

Section of Page with the issue

Why is this a problem?

Suggested Fix

[github-actions]

Heads up @WordPress/docs-issues-coordinators, we have a new issue open. Time to use 'em labels.

[github-actions]

Heads up @kafleg @carolinan @TeBenachi - the "themes" label was applied to this issue.

[justintadlock]

I can take this one on.

[justintadlock]

Notice added:

As of WordPress 6.4, attachment pages are no longer enabled by default on new installations. Users can enable them with a plugin, so it is still good practice to test your theme and ensure it properly displays content when viewing an attachment page.