docusaurus docs:version overwrite option

## 🚀 Feature

Add some `--force` or `--overwrite` flag to `docusaurus docs:version` command.

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

Sure.

## Motivation

Currently, the `docusaurus docs:version` command fails if a version is already exists, but for version range like 1.x, it might be convenient to simply replace the whole 1.x content with the `next` version.