docs: write initial documentation

docs/packaging.md

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+# Packaging Nixlets
+
+## GitLab Package Registry
+
+To package and upload Nixlets to the GitLab Package Registry there is a helper script available.
+Use it like so:
+
+```nix title="flake.nix"
+apps.upload = {
+  type = "app";
+  program = pkgs.callPackage nixlet-lib.uploadNixletsToGitlab {
+    projectId = "<GitLab Project ID>";
+    nixlets = [
+      # list of Nixlets
+    ];
+  };
+};
+```
+
+You can then run this:
+
+```sh
+nix run .#upload --impure
+```
+
+`--impure` is needed because the Script needs to access the env variable `AUTH_HEADER`.
+
+`AUTH_HEADER` needs to contain the Header GitLab expects for auth.
+In GitLab CI this should for example be `JOB-TOKEN: $CI_JOB_TOKEN`.
+A personal access token requires this format instead: `PRIVATE-TOKEN: <your token>`.
+
+!!! note
+    The script only uploads a version once. If the version already exists it will skip that Nixlet.
+
+## General
+
+Nixlets are self contained and receive all their dependencies etc. from the
+caller rendering it.
+
+This means that you can just compress any Nixlet directory to a tarball, upload
+it somewhere and then fetch it later where you intend to use/render it
+(the upload script for GitLab CI does basically that).
+
+See [Usage](./usage.md) for information on how to fetch and render it later.