docs(nix): add startup guide and clarify addLoginEntry

- Replace 'Extra options' with greetd and getty autologin examples
- Add bash/zsh and fish variants for getty autologin
- Add link to Nix Module Options reference
- Clarify addLoginEntry only has effect when a DM is configured

nix/hm-modules.nix

@@ -102,7 +102,7 @@ in
         description = ''
           Mango configuration written in Nix. Entries with the same key
           should be written as lists. Variables and colors names should be
-          quoted. See <https://mangowc.vercel.app/docs> for more examples.
+          quoted. See <https://mangowm.github.io/docs> for more examples.
 
           ::: {.note}
           This option uses a structured format that is converted to Mango's

nix/nixos-modules.nix

@@ -12,7 +12,7 @@ in {
       addLoginEntry = lib.mkOption {
         type = lib.types.bool;
         default = true;
-        description = "Whether to add a login entry to the display manager for mango";
+        description = "Whether to add a login entry to the display manager for mango. Only has effect if a display manager is configured (e.g. SDDM, GDM via `services.displayManager`).";
       };
       package = lib.mkOption {
         type = lib.types.package;