* More prep
* rename xxx-api to api-xxx
* move content around for plugins and themes
* wip docs: using themes
* docs(v2): tweak using plugins
* docs(v2): list official themes in docs
* docs(v2): advanced themes
* wip notes for lifecycle apis
* resolve PR review discussions
* lower case "theme"
* better intro for using themes
* add a simple README to @docusaurus/theme-classic
* remove list of components from theme classic README and replace with link to directory