Add core plugins docs to cli.vuejs.org

[NataliaTepluhina]

What problem does this feature solve?

We currently don't have plugins documented directly on the documentation website, but simply link to their README files on GitHub on the dev branch.

We should dedicate some time to create some minimal docs for each core plugin on the documentation website to prevent such confusion in the future.

#4069 (comment)

What does the proposed API look like?

In my opinion, we need to create a new section on Vue CLI website with a list of core plugins, where every core plugin chapter contains documentation currently shown on README files. Maybe it's also a good moment to improve plugins docs.