How-To: Write and Enable a Nexus Fusion Plugin

To ease the development of plugins for users, we should provide an example plugin (e.g. simple image or metadata) in the docs.

- explain what plugins are
- create a code sandbox project for the plugin
- use case/example: data download (from the nexus-web-plugins repo)
- create the react component to satisfy the use case
- build and export the artifacts
- paste the artifacts to the right location (e.g. fusion plugin directory)
- document the process to configure a plugin for a deployed instance
- adapt the plugin manifest
- point to the public nexus plugin repo for the actual plugin and more examples

Let's do it during internal onboarding.

Start by reading the data-download example in the nexus-web-plugins repository and reviewing the existing Fusion plugin directory and deployment configuration. Create an example plugin project, document its React component, build/export steps, manifest changes, installation, and configuration for a deployed instance. Done means the example and onboarding guide cover the full process and point to the public plugin repository.

Written by the indexing model from the issue text.