Improve search engine description, listing and urls (SEO optimization)


1. Description could be a bit better, right now it's using page's content
2. ~Link to `/docs` brings to a 404: https://game.ci/docs we can probably redirect this to `/docs/introduction`~
3. Improve sharing description and image. [Facebook Debugger](https://developers.facebook.com/tools/debug/?q=https%3A%2F%2Fgame.ci) says there is no `og:image` tag so the sharing result on Facebook looks like this:

I assume sharing on other platforms such as slack and discord will result in something similar.

Start by inspecting the metadata for the game.ci homepage and the /docs entry point. Compare the current description and sharing preview with the Facebook Debugger, then verify that the final metadata includes an improved description, sharing image, and appropriate documentation URL behavior.

Written by the indexing model from the issue text.