ENH: add clone button to lectures for quick access to nb repo

[aadsouza]

for example with the datascience lectures the options would be

[Open repository](https://github.com/QuantEcon/lecture-datascience.notebooks)
[Copy clone command to clipboard](git clone https://github.com/QuantEcon/lecture-datascience.notebooks)
[Open in GitHub Desktop](x-github-client://openRepo/https://github.com/QuantEcon/lecture-datascience.notebooks)

addresses https://github.com/QuantEcon/lecture-datascience.myst/pull/83#issuecomment-842817971

[aadsouza]

@mmcky ideally the clone button could support both jupyterlab and jupyter notebook upon launch.