DOC: Convert README and unmaintained-images from rst to markdown

Conversation done using pandoc and manually tweaked to fix indentation, headers
and spacing as well as introducing GitHub alerts[^1]

```
$ pandoc -f rst -t gfm README.rst -o README.md
$ pandoc -f rst -t gfm unmaintained-images.rst -o unmaintained-images.md
```

[^1]: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts

Author: jcfr

README.md

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+# Docker images for 3D Slicer
+
+## Images
+
+- slicer/slicer-notebook
+  [![slicer-notebook-images](https://img.shields.io/docker/image-size/slicer/slicer-notebook/latest)](https://hub.docker.com/r/slicer/slicer-notebook)
+  Ready-to-run Docker images containing Slicer and Jupyter. See [usage](#usage-of-slicer-notebook-image) information below.
+
+- slicer/slicer-base
+  [![slicer-base-images](https://img.shields.io/docker/image-size/slicer/slicer-base/latest)](https://hub.docker.com/r/slicer/slicer-base)
+  The image used to build Slicer each time a Pull Request is submitted.
+  For more details, see [GitHub Actions workflow](https://github.com/Slicer/Slicer/tree/main/.github)
+
+### Unmaintained Images
+
+Information about unmaintained images are available [here](unmaintained-images.md).
+
+## Usage of `slicer-notebook` image
+
+> [!WARNING]
+> `slicer/slicer-notebook` images on dockerhub are outdated, therefore the
+> examples below uses latest images from `lassoan/slicer-notebook`.
+
+1. Start a Jupyter server by running this command:
+
+    Linux or macOS:
+
+    ```
+    docker run -p 8888:8888 -p 49053:49053 -v "$PWD":/home/sliceruser/work --rm -ti lassoan/slicer-notebook:latest
+    ```
+
+    Windows:
+
+    ```
+    docker run -p 8888:8888 -p 49053:49053 -v "%USERPROFILE%":/home/sliceruser/work --rm -ti lassoan/slicer-notebook:latest
+    ```
+
+2. Open the link shown at the end of the command-line output (`http://127.0.0.1:8888/?token=...`) in a web browser.

README.rst

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+# Unmaintained Images
+
+In the process of improving the Slicer infrastructure, these images are
+not updated anymore. Only the `slicer/slicer-base` image is used for
+continuous integration with [GitHub Actionsworkflow](https://github.com/Slicer/Slicer/tree/main/.github).
+
+  - slicer/slicer-dependencies
+    [![slicer-dependencies-images](https://img.shields.io/docker/image-size/slicer/slicer-dependencies/latest)](https://hub.docker.com/r/slicer/slicer-dependencies)
+    An image containing all the dependencies to build Slicer itself:
+    ITK, VTK, CTK, Qt, etc.
+
+  - slicer/slicer-build
+    [![slicer-build-images](https://img.shields.io/docker/image-size/slicer/slicer-build/latest)](https://hub.docker.com/r/slicer/slicer-build)
+    An image containing a Slicer build tree along with all the
+    dependencies to build Slicer.
+
+  - slicer/slicer-test
+    [![slicer-test-images](https://img.shields.io/docker/image-size/slicer/slicer-test/latest)](https://hub.docker.com/r/slicer/slicer-test)
+    An image containing a Slicer build and test tree along with all the
+    dependencies to build Slicer.
+
+  - slicer/slicer-test:opengl
+    [![slicer-test-opengl-images](https://img.shields.io/docker/image-size/slicer/slicer-test/opengl)](https://hub.docker.com/r/slicer/slicer-test)
+    An image based on thewtex/opengl:centos which contains configuration
+    files to run Slicer's tests
+
+# Usage
+
+To build and package a local Slicer source tree at `~/src/Slicer` 
+against pre-built dependencies:
+
+```
+docker run --name slicer slicer/slicer-dependencies
+# Copy the generated Slicer package to the /tmp/ directory.
+docker cp slicer:$(docker cp slicer:/usr/src/Slicer-build/Slicer-build/PACKAGE_FILE.txt - | tar xO) /tmp/
+docker rm slicer
+```
+
+# Update
+
+To update the Slicer revision, first download Slicer sources:
+
+```
+git clone https://github.com/Slicer/Slicer
+```
+
+Then download SlicerDocker sources and check out a local branch for the update:
+
+```
+git clone [email protected]:Slicer/SlicerDocker
+cd SlicerDocker
+git checkout -b update-$(date +%F)
+```
+
+And run the update script:
+
+```
+./slicer-base/update.sh /path/to/src/Slicer
+```