feat: add content freshness agent skill and 7.4 release notes

[Raghaddahi]

PR Description

Description

This PR adds the Content Freshness Skill to help audit the Wagtail User Guide after new releases. It uses an AI agent to compare release notes against existing guide content to find outdated pages.

Key Changes

• content-freshness.md: Instructions for the AI to filter and prioritize updates.
• 7.4.md: The latest release data for testing.

Testing

I tested it with in vscode with copilt
and that was the result of using it on the 7.4 version

• Wagtail 7.4 Content Freshness Analysis

Summary of the 7.4 Results

| Priority | Page Title | Reason for Update |
| :--- | :--- | :--- |
| **MEDIUM** | [Manage Pages](https://guide.wagtail.org/en-latest/how-to-guides/manage-pages/) | New StreamField deferred validation options. |
| **MEDIUM** | [Manage Images](https://guide.wagtail.org/en-latest/how-to-guides/manage-images/) | Alt text auto-population behavior change. |
| **MEDIUM** | [Find your way around](https://guide.wagtail.org/en-latest/how-to-guides/find-your-way-around/) | Concurrent editing UI and idle user indicators. |
| **MEDIUM** | [Action Bar](https://guide.wagtail.org/en-latest/how-to-guides/manage-pages/) | New logic for hiding "Add child page" button. |
| **LOW** | [Concepts / oEmbed](https://guide.wagtail.org/en-latest/reference/) | Support for Flourish and Heyzine oEmbeds. |

**Total estimated effort:** ~4.5 hours for a full guide refresh.

AI Usage

AI was used to help structure the agent instructions and summarize this PR description.