docs: use relative links
    
    Allows for links to work both on systemd.io (or forks) and
    when viewed on https://github.com/systemd/systemd/tree/main/docs
    
    Note that the markdown links are converted by jekyll-relative-links[1]
    to html. This plugin is enabled by default on github pages[2][3].
    
    Due to a bug in jekyll-relative-links – see
    https://github.com/benbalter/jekyll-relative-links/issues/61 –
    we need to avoid line-wrapped links when using relative markdown links.
    
    [1] https://github.com/benbalter/jekyll-relative-links
    [2] https://github.blog/2016-12-05-relative-links-for-github-pages/
    [3] https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/about-github-pages-and-jekyll#plugins