docs: add remaining docs back with some additions/improvements

docs/usage.md

@@ -0,0 +1,73 @@
+# Usage
+
+## Usage (with flake-parts)
+
+```nix
+# flake.nix
+{
+  ...
+  inputs.nix-gitlab-ci.url = "gitlab:TECHNOFAB/nix-gitlab-ci/<version>?dir=lib";  # recommendation: pin to the latest release/version
+  
+  outputs = {...}: flake-parts.lib.mkFlake {...} {
+    imports = [
+      inputs.nix-gitlab-ci.flakeModule
+    ];
+    ...
+    
+    perSystem = {pkgs, ...}: {
+      ci = {
+        config = {
+          # configure Nix-GitLab-CI here, see docs for options
+        };
+        pipelines."default" = {
+          stages = ["test"];
+          jobs = {
+            "test" = {
+              stage = "test";
+              nix.deps = [pkgs.unixtools.ping];
+              script = [
+                "ping -c 5 8.8.8.8"
+              ];
+            };
+          };
+        };
+        # runs on a merge request for example
+        pipelines."merge_request_event" = {
+          stages = ["some_stage"];
+          jobs = { ... };
+        };
+      };
+      ...
+    }
+  }
+}
+```
+
+Now either use this in your .gitlab-ci.yml or setup Soonix to auto generate this
+file for you with the right version (see the [docs][docs-soonix] for more).
+
+```yaml
+# .gitlab-ci.yml
+include:
+  - component: gitlab.com/TECHNOFAB/nix-gitlab-ci/nix-gitlab-ci@<version>  # recommendation: pin to the latest release/version (don't use "main" etc.)
+    inputs:
+      version: <version>  # docker image tag, use the same version as a above
+```
+
+## Usage (directly)
+
+```nix
+let
+  cilib = inputs.nix-gitlab-ci.lib {inherit pkgs;};
+in
+  cilib.mkCI {
+    config = ...;
+    pipelines."default" = ...;
+  };
+  # exposes `soonix` for the soonix hook and `packages` which contain the configs, jobs etc.
+```
+
+______________________________________________________________________
+
+Since V2 multiple pipelines are supported.
+See [Multiple Pipelines](./multi_pipeline.md) for more.