chore(docs): change permalink at the Jekyll config

Some of the internal links didn't work.
To make them work we had two options:

1. Keep the trailing slash at the jecyll config:
The built structure of the html files is one directory for each file, including a html file.
This is why a relative link including the `../` is required to get out of the built folder and make it work.
e.g.
config: permalink: /:title/
HACKING.md: [Coding Style](../CODING_STYLE)

2. Remove the trailing slash, the built structure is like the structure in dev: one html file for each markdown file.
In this case, the links as they are implemented just work.
e.g.
config: permalink: /:title
HACKING.md: [Coding Style](CODING_STYLE)

We use option 2 :)

docs/_config.yml

@@ -4,7 +4,7 @@ title: systemd
 baseurl: "" # the subpath of your site, e.g. /blog/
 url: "https://systemd.io" # the base hostname & protocol for your site
 
-permalink: /:title/
+permalink: /:title
 
 # Build settings
 markdown: kramdown