The "How to create a plugin" doc needs an update

[hezyin]

Documentation Scope

We've updated the interface of plugins in the last major release so the "How to create a plugin doc" also needs an update.

The doc is currently living at: https://github.com/merico-dev/lake/blob/main/plugins/README.md

We might want to take this opportunity to move it to the docs folder and migrate to our docs website as well.