Update "Adding Report" document for `ReportView`

[chitvs]

Problem:

This PR addresses the lack of documentation for creating reports for snippets and non-page models in Wagtail. The existing documentation primarily focuses on generating reports for pages, leaving a gap for developers looking to implement reports for other model types.

Issue Number:

This PR fixes #12428.

Changes Made:

• Added documentation that outlines how to create custom reports for snippets and non-page models.
• Provided code examples demonstrating how to implement the UnpublishedChangesReportView to handle both pages and snippets.
• Included instructions on setting up the menu item and permission restrictions for the new report.

Motivation:

The aim of these changes is to assist developers in understanding how to create custom reports beyond just pages, facilitating a more versatile use of the Wagtail reporting system.

Related Documentation:

• Wagtail documentation on adding reports
• Reference to the GitHub issue for context and discussion.

[squash-labs[bot]]

Manage this branch in Squash

Test this branch here: https://chitvsadd-reports-for-snippets-dk8ko.squash.io

[chitvs]

ok, completely fine.

[lb-]

See comment https://github.com/wagtail/wagtail/issues/12428#issuecomment-2427843563