docs: add remaining docs back with some additions/improvements

docs/soonix.md

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+# Soonix Integration
+
+[Soonix](https://soonix.projects.tf) can be used to automatically generate the
+`.gitlab-ci.yml` for you.
+This will by default include the CI/CD component with the same version as the
+flake (using the `VERSION` file in this repo).
+
+You can specify some options to configure this, like changing the component URL
+or adding extra data to your `.gitlab-ci.yml` (like this repo does to bootstrap
+Nix-GitLab-CI). See [Options](./options.md#configsoonix) for all the config options.
+
+You can use it like this:
+
+```nix
+let 
+  ci = cilib.mkCI { ... };
+in {
+  soonix.hooks."ci" = ci.soonix;
+}
+```