Allow additional plugin examples to be registered

### Problem

Some extensions are _themselves_ extensible (e.g. [jupyterlab-markup](https://github.com/agoose77/jupyterlab-markup), [jupyterlab-lsp](https://github.com/jupyter-lsp/jupyterlab-lsp)), and might like to advertise this in a more declarative way, especially in documentation/demo settings (e.g. #16).

### Proposed Solution

Offer a new launcher card per template.

Extend the public api proposed on #49 to allow registering new plugin templates, with a few options including a documentation markdown file.

### Additional context

#### Types

```ts
export interface IPluginExample {
title: string;
description: string;
src: () => Promise;
doc?: () => Promise;
icon?: LabIcon;
filename?: str;
}
export interface IPluginPlayground {
addExample(example: IPluginExample): void;
}
```