#774: Add Attachment Page Template

[MiguelAxcar]

This pull request introduces a new template part for displaying attachment pages, allowing a more consistent and customized view for attachments within the theme.

Changes included

• A new file content-attachment.php that contains the HTML structure and PHP logic for rendering attachment pages.

Testing instructions

• Navigate to a page which has an attachment, and the link is set for something like https://wikimediadiff.vipdev.lndo.site/?attachment_id=89030
• Verify that the new attachment page template is loaded and that it displays the attachment correctly.
• Confirm that the styling and layout match the rest of the theme.

Screenshot

[ckoerner]

Confirming that this is working as intended in production. Across languages and consistently through the archives. Hurrah!