docs: unify markdown files with documentation titles

This is preparational work to reduce the surface are of the different mappings

We want to reduce the following:

/guides/secrets.md -> Guides/getting started/How to use flakes with sops -> with '# Secrets Management'

- Title
- URI
- Filepath

should follow a stricter pattern to make it easy to maintain

docs/site/guides/getting-started/add-services.md

@@ -1,5 +1,3 @@
-# How to add services
-
 A service in clan is a self-contained, reusable unit of system configuration that provides a specific piece of functionality across one or more machines.
 
 Think of it as a recipe for running a tool — like automatic backups, VPN networking, monitoring, etc.