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

templates/clan/flake-parts/flake.nix

@@ -20,7 +20,7 @@
         inputs.clan-core.flakeModules.default
       ];
 
-      # https://docs.clan.lol/guides/getting-started/flake-parts/
+      # https://docs.clan.lol/guides/flake-parts
       clan = {
         imports = [ ./clan.nix ];
       };