Document how to use external ORT plugins

[mmurto]

It's possible to write ORT plugins externally to ORT repo (for example https://github.com/oss-review-toolkit/ort-package-manager-plugin), but it is not instantly obvious how to actually use these plugins with ORT. It should probably be documented somewhere.

[mmurto]

It would be helpful if this part of documentation also contained information on how to use the published Docker images as base and add a custom plugin to create a new image, if such thing is possible.