Docusaurus: feat(v2): home page for docs

[yangshun]

🚀 Feature

When I navigate to https://v2.docusaurus.io/docs, I see empty page, not even a sidebar on the left. It would be great if we have a way to define content for this page e.g. docs/index.md

Refer to https://v2.docusaurus.io/feedback/p/home-page-for-docs

Have you read the Contributing Guidelines on issues?

Yes

Pitch

This can be specified via an option on the docs plugin.

[mohamedsgap]

I think we should make a list with the latest 5 versions of docs, so it will be easy to navigate between them, and also write an introduction about the whole doc, and some features of the last shipped version of the doc too
@yangshun

[yangshun]

Not every docs have versioning. People can already built it themselves like how we do it on https://v2.docusaurus.io/versions

[smyrick]

Maybe instead it should just redirect to the "real" home page https://v2.docusaurus.io/docs/introduction

[vinibalbino]

I created this simple design thinking of being the home page for the docs page. for the user to select the version he wants to use. could it be a solution?