Add instructions for actually running the tutorials to intro.html

[hyanwong]

We should make it easy for a tutee to copy and paste the code cells in most of the tutorials to their own Jupyter notebooks (especially the basics/getting_started ones), and for it to work the majority of the time. For this we need to give them instructions about what they need to install.

One possibility is e.g. to expose requirements.txt to a general location and in intro.html recommend something like

pip install -r https://tskit.dev/tutorials/requirements.txt

Although this would install jupyterbook, which is not actually needed to run the code cells. To run the R stuff we also need the tutee to run R -e 'install.packages(c("reticulate", "IRkernel")); IRkernel::installspec()' as described in the GitHub repo README file.

Also, basically the first code line in basics.html is ts = tskit.load("data/basics.trees") which won't work if copied and pasted, so this is a prime candidate for loading from a URL (see #116 & tskit-dev/tskit#1566)