docs: use relative links

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