Replace value of ogp_image in conf.py to an image included in the repository instead of assets manager

In conf.py ([line 87 in main branch](https://github.com/canonical/sphinx-docs-starter-pack/blob/main/docs/conf.py#L87)), we have:

```bash
ogp_image = "https://assets.ubuntu.com/v1/cc828679-docs_illustration.svg"
```

While elsewhere, we advise not to use the asset manager for images in our RTD docs and to include the images in the GH repository instead. I find that teams using the starter pack tend to follow the example and use assets.ubuntu.com. We should show a better example by including the image file in the starter pack repository and pointing to it.