Add repo to documentation

[josefnorlin]

Describe the bug
Great work! Reading your guide here: https://docs.react-firebase.com/getting-started#getting-started
You say that the first step is to clone your repo. But there's no link to the repo: https://github.com/CreateThrive/react-firebase-admin
(Took a while to find this repo, and also I got stuck in very similar projects on the way to find it here)

To Reproduce
Steps to reproduce the behavior:

1. Go to https://docs.react-firebase.com/getting-started#getting-started
2. On getting started, read "Clone repo"
3. But you find no link to the repo