docs: write the 0.20.0 changelog

The changelog page has been an empty stub carrying only a title, and was
hidden from the sidebar. Fill it in from the 38 pull requests merged since the
0.19.0 tag, grouped by what a reader would look for, and link the GitHub
release notes for the full per-PR list and the upgrade guide for the steps an
existing deployment has to take. Earlier releases are not backfilled.

Now that the page has content, show it in the sidebar.

docs/content/_meta.js

@@ -13,8 +13,5 @@ export default {
     "href": "https://gptcloud.arc53.com/"
   },
   "Guides": "Guides",
-  "changelog": {
-    "title": "Changelog",
-    "display": "hidden"
-  }
+  "changelog": "Changelog"
 }

docs/content/changelog.mdx

@@ -1,3 +1,93 @@
 ---
 title: 'Changelog'
----
+description: What changed in each DocsGPT release, with links to the upgrade steps an existing deployment needs.
+---
+
+import { Callout } from 'nextra/components'
+
+# Changelog
+
+The notable changes in each release. Every release on GitHub also carries
+[auto-generated notes](https://github.com/arc53/DocsGPT/releases) listing every merged pull
+request, and [Upgrading](/upgrading) covers the steps an existing deployment has to take.
+
+## 0.20.0
+
+### DocsGPT installs from PyPI
+
+The backend is published as [`docsgpt`](https://pypi.org/project/docsgpt/): the API server, the
+web UI, the Celery worker and the maintenance scripts in one package, behind a single `docsgpt`
+command. `pip install docsgpt` then `docsgpt api` serves the API and the UI on one port, and
+`docsgpt worker` runs the worker. The optional engines are extras, `docsgpt[docling]` and
+`docsgpt[milvus]`. See [Install with pip](/Deploying/Pip-Install).
+
+### The Python package is now `docsgpt`
+
+The import package was renamed from `application` to `docsgpt`, the name it has on PyPI. Entry
+points move with it: `celery -A docsgpt.app.celery worker` and
+`uvicorn docsgpt.asgi:asgi_app`. The old spellings still run for this release and print a
+`FutureWarning`, and Celery tasks queued under the old names are still consumed. The
+[upgrade guide](/upgrading) has the details.
+
+### Smaller default install and images
+
+The document and vector-store engines that pulled the heaviest dependencies are now extras
+rather than defaults, so a stock install no longer carries PyTorch or the CUDA stack. The
+published image follows the same split: the default `arc53/docsgpt` image is the slim one, and
+`arc53/docsgpt:<version>-docling` bakes in the docling parser engine, its models and tesseract
+for OCR.
+
+### Faster embeddings, pinned to your installation
+
+Embeddings run through [FastEmbed](https://github.com/qdrant/fastembed) on ONNX Runtime. Query
+embedding now happens on the Celery worker rather than in the API process, which keeps the API
+small; a worker consuming the `embeddings` queue is required for search. The embedding model is
+pinned to the installation instead of following the release, so an upgrade never silently
+changes the model behind an existing index, and a query against an index built with a different
+model is now warned about. New installs default to
+`ibm-granite/granite-embedding-311m-multilingual-r2`, which is multilingual with a 32k-token
+context. The `reembed` script rebuilds vectors in place from the chunk text already stored.
+
+<Callout type="warning">
+  If you start your worker with an explicit `-Q`, add the `embeddings` queue. See
+  [Upgrading](/upgrading) for the one-line change and what happens without it.
+</Callout>
+
+### Broader document support
+
+Parsing gained support for more document, spreadsheet, presentation, EPUB, XHTML and image
+formats, along with a reworked OCR path. Parsing behaviour is configurable: markdown conversion,
+structured output, table reconstruction and improved PDF handling.
+
+### Agents and workflows
+
+Agents no longer require a source. The synthetic "Default" source is gone, and an agent with no
+source or retriever is a valid agent that lists and answers normally. Workflow agents can be
+exported and imported as files, so a workflow can move between deployments or into version
+control.
+
+Artifact and tool-call handling was hardened throughout: durable tasks retry rather than lose
+work on resume, tool calls are validated more strictly, and sandbox sessions are steadier.
+
+### Chat and attachments
+
+Cross-turn chaining against the Responses API is now bounded, conversation compression persists
+across turns instead of being recomputed, and requests carry prompt-cache hints. Attachments
+carry provenance through parsing, unparseable chat attachments are refused at upload rather than
+failing mid-answer, and the composer guards against sending while an attachment is still
+processing.
+
+### Widget
+
+The React widget got a UI refresh and an expand and collapse toggle, and was brought back in
+line with the current API contract. The npm packages `docsgpt` and `docsgpt-react` are published
+at 0.7.1.
+
+### Also in this release
+
+- The code-execution sandbox runner is published as `arc53/docsgpt-sandbox`, so the Kubernetes
+  manifest no longer needs an image you build yourself.
+- Releases publish the Docker images and the PyPI package from the release workflow.
+- The [architecture guide](/Guides/Architecture) was rewritten.
+- Markdown code spans are no longer corrupted by citation rendering.
+- `setup.ps1` runs on Windows PowerShell 5.1 again.