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), we have:

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.