Added introduction to Bayesian optimisation notebook.

Also fixed some links which had broken after renaming the old kernels
notebook in a previous PR.

Added `sudo apt-get update` command before `sudo apt-get install` as
recommended in
https://docs.github.com/en/actions/using-github-hosted-runners/customizing-github-hosted-runners
in order to mitigate package installation failures.

Incorporated feedback into BO notebook and added link to BO notebook
from project README.