docs: add `package-info.java` and `readme.md`

For `API/CheerpJ`

src/com/plantuml/api/cheerpj/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Provides classes used to manage PlantUML with the 
+ * <a href="https://labs.leaningtech.com/cheerpj3" target="_top">
+ * CheerpJ</a> API.
+ * 
+ */
+package cheerpj;

src/com/plantuml/api/cheerpj/readme.md

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+# Directory Documentation for `cheerpj`
+
+## Description
+This package provides classes used to manage [CheerpJ](https://labs.leaningtech.com/cheerpj3) API.
+
+## Reference
+- [CheerpJ](https://labs.leaningtech.com/cheerpj3)
+
+## Use by
+- :octocat: [plantuml/plantuml-core](https://github.com/plantuml/plantuml-core)
+  - on [`/plantuml-core/src/main/java`](https://github.com/plantuml/plantuml-core/tree/main/plantuml-core/src/main/java)
+- :octocat: [plantuml/plantuml.js](https://github.com/plantuml/plantuml.js)
+
+## Demonstration
+- [`plantuml-core/raw.html`](https://plantuml.github.io/plantuml-core/raw.html)
+- [`plantuml.js`](https://plantuml.github.io/plantuml.js/)