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

[GabLeRoux]

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 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.

[GabLeRoux]

The following plugin might be helpful for the redirection:
https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-client-redirects

[GabLeRoux]

There used to be a ogImage for Docusaurus v1. According to documentation for v2, this is now handled in theme configuration:
https://docusaurus.io/docs/migration/manual#footericon-copyright-ogimage-twitterimage-docssidenavcollapsible

Documentation for og:image:
https://docusaurus.io/docs/api/themes/configuration#meta-image

Documentation for metadata (site description, etc.):
https://docusaurus.io/docs/api/themes/configuration#metadata

[webbertakken]

Nice catch!

Actually I think perhaps "introduction" should just link to /docs. We don't need a plugin for that. Note that if google links to a redirect (3xx) instead of a real page (2xx) it'll be bad for your SEO ranking. Either way, that would be solved when google reindexes the page I think.

I think I'd prefer to just have /docs/introduction be docs/index instead so that /docs works again.

[webbertakken]

Fixed the 404.

I think if we're going to do SEO it might make sense to also install PWA. I have a config for that which already includes some meta stuff and icons.