[DOC]: Change "Get Started" button on Home

### 📚 The doc issue

I'd suggest to change the home page at https://colossalai.org/, so that the "Get Started" button points to https://colossalai.org/docs/get_started/installation instead of the downloads page. The way it is now, newbies get stuck, because although they now where to download Colossal-AI, they don't know how to use it.