docs: add a "front matter" snippet to our markdown pages

It turns out Jekyll (the engine behind GitHub Pages) requires that pages
include a "Front Matter" snippet of YAML at the top for proper rendering.

Omitting it will still render the pages, but including it opens up new
possibilities, such as using a {% for %} loop to generate index.md instead of
requiring a separate script.

I'm hoping this will also fix the issue with some of the pages (notably
CODE_OF_CONDUCT.html) not being available under systemd.io

Tested locally by rendering the website with Jekyll. Before this change, the
*.md files were kept unchanged (so not sure how that even works?!), after this
commit, proper *.html files were generated from it.

docs/CODE_OF_CONDUCT.md

@@ -1,3 +1,7 @@
+---
+title: The systemd Community Conduct Guidelines
+---
+
 # The systemd Community Conduct Guidelines
 
 This document provides community guidelines for a safe, respectful, productive, and collaborative place for any person who is willing to contribute to systemd. It applies to all “collaborative spaces”, which is defined as community communications channels (such as mailing lists, submitted patches, commit comments, etc.).