Add local ansible Galaxy repositories

docs/ansible/local.md

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+# Local ansible repository
+
+A local `ansible` repo is a Galaxy v3 server: `ansible-galaxy collection publish`
+and `ansible-galaxy collection install` work against `/api/v1/local/<repo>`.
+`publish` needs an API key; any value works.
+
+```bash
+ansible-galaxy collection build
+ansible-galaxy collection publish unkin-hello-1.0.0.tar.gz \
+  --server https://artifactapi.example.com/api/v1/local/<repo>/ --api-key unused
+ansible-galaxy collection install unkin.hello \
+  --server https://artifactapi.example.com/api/v1/local/<repo>/
+```
+
+Publishing reads `collection_info` from the tarball's `MANIFEST.json` and
+finishes the import synchronously, so the import task is already `completed`.
+Uploads over 256 MiB, tarballs with members or symlinks escaping the collection root, or
+with an invalid name, version or dependency map are rejected with 400.
+
+The repo serves `api/v3/collections/<ns>/<name>/`, the paginated
+`versions/` list (`limit` up to 100, `offset`), `versions/<version>/` with
+`download_url`, `artifact.sha256` and `metadata.dependencies`, and
+`artifacts/<ns>-<name>-<version>.tar.gz`.
+
+Versions are immutable: publishing an existing `namespace.name:version` is
+rejected with 409.