Doc: Layout fix in index.md
-| [Homepage](https://github.com/tum-pbs/PhiFlow) | [API](phi) | [Demos](https://github.com/tum-pbs/PhiFlow/tree/develop/demos) | [<img src="https://www.tensorflow.org/images/colab_logo_32px.png" height=16> Fluids Tutorial](https://colab.research.google.com/drive/1S21OY8hzh1oZK2wQyL3BNXvSlrMTtRbV#offline=true&sandboxMode=true) / [Playground](https://colab.research.google.com/drive/1zBlQbmNguRt-Vt332YvdTqlV4DBcus2S#offline=true&sandboxMode=true)|
-| :------ | :-------- | :-------- | :------ |
+[**Homepage**](https://github.com/tum-pbs/PhiFlow)
+&nbsp;&nbsp;&nbsp; [**API**](phi)
+&nbsp;&nbsp;&nbsp; [**Demos**](https://github.com/tum-pbs/PhiFlow/tree/develop/demos)
+&nbsp;&nbsp;&nbsp; [<img src="https://www.tensorflow.org/images/colab_logo_32px.png" height=16> **Fluids Tutorial**](https://colab.research.google.com/drive/1S21OY8hzh1oZK2wQyL3BNXvSlrMTtRbV#offline=true&sandboxMode=true)
+&nbsp;&nbsp;&nbsp; [<img src="https://www.tensorflow.org/images/colab_logo_32px.png" height=16> **Playground**](https://colab.research.google.com/drive/1zBlQbmNguRt-Vt332YvdTqlV4DBcus2S#offline=true&sandboxMode=true)
If you would like to get right into it and have a look at some code, check out the
[tutorial notebook on Google Colab](https://colab.research.google.com/drive/1S21OY8hzh1oZK2wQyL3BNXvSlrMTtRbV#offline=true&sandboxMode=true).
