Add Google colab buttons for Colab option

[mmcky]

@HumphreyYang @Smit-create @HengchengZhang

I think we should add a small colab button to each lecture:

The raw html

<div align="right"><button><a href="https://colab.research.google.com/github/QuantEcon/workshop.africa-july2023/blob/main/day-01/exercise_set_1.ipynb"><img src="" heght="10px"/><img
  src="https://colab.research.google.com/assets/colab-badge.svg"
  alt="open with Colab" width="100px"/></a></button></div>

and for the ones that require collab we use

the raw html

<br/>
<div align="center"><button><a href="https://colab.research.google.com/github/QuantEcon/workshop.africa-july2023/blob/main/day01/exercise_set_1.ipynb"><img src="" heght="20px"/><img
  src="https://colab.research.google.com/assets/colab-badge.svg"
  alt="open with Colab" width="200px"/></a></button></div>
<br/>

This will give students the option to open that notebook in colab if they prefer.

[mmcky]

The important part is to update the path to the notebook in GitHub for each badge

[mmcky]

Placement:

I think they would work well as the second line below the title

[mmcky]

Testing: Please test the links 🙏

[HumphreyYang]

Hi team,

Links for week one are added. Please kindly follow this in future exercises (CC @shlff @DiaPorntipa @HengchengZhang).