Fix failing doc pipeline for assets in README.md

Switch to MyST’s Markdown include directive to fix the failing doc
pipeline when `README.md` contains a relative path to an asset, e.g.:

```md
![MyProject icon](./share/icon/myproject-512x512.png)
```

Author: claui

{{ cookiecutter.pypi_package_name }}/doc/sphinx/index.md

@@ -0,0 +1,14 @@
+# Documentation for {{ cookiecutter.project_title }}
+
+```{toctree}
+:maxdepth: 2
+:caption: Contents
+
+usage
+autoapi/index
+```
+
+```{include} ../../README.md
+:relative-docs: true
+:relative-images: true
+```

{{ cookiecutter.pypi_package_name }}/doc/sphinx/index.rst

@@ -0,0 +1,6 @@
+# Usage
+
+```{include} ../../USAGE.md
+:relative-docs: true
+:relative-images: true
+```