Docs site: name the maintainer in the footer, not the repository owner

Cayman's footer renders "<repository> is maintained by <site.github.owner_name>",
and that name comes from whoever owns the repository, so seredis.dev read
"StackExchange.Redis is maintained by StackExchange". That is a statement about
ownership rather than maintenance, and it isn't true of this project.

jekyll-github-metadata merges anything set under `github:` in _config.yml over
the values it reads from the API (SiteGitHubMunger#github_namespace is
`drop.merge(@original_config)`), so overriding owner_name/owner_url is enough -
no layout override, and no copy of the theme's markup to keep in step. Nothing
else on the site reads either key; Cayman uses them only in that footer span.

The alternative was dropping the line entirely via `is_project_page: false`, but
Cayman guards its "View on GitHub" button with the same flag, which we do want.