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

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.