[Documentation enhancement request] `docs/getting-started.md` is outdated

Feature description
---------
Doc [docs/getting-started.md](https://github.com/doccano/doccano/blob/master/docs/getting-started.md) doesn't reflect recent documentation changes.

Propose: replace its content with [README.md's Usage](https://github.com/doccano/doccano/blob/master/README.md#usage) link.