This first commit is a rebase of Eric's initial PR as of db1ce62 onto
`main`, fixing up some changes caused by the CI infrastructure changing
a bit since the PR was first opened.

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

This moves much of the fetch- and process-related code into separate
scripts that assert far more about the directory structure, and fail if
they do not match the assumptions.  We don't want to accidentally get
out-of-sync while we're changing things and end up with a tutorials job
that isn't really doing its job without us noticing.

The tutorials-fetching script can now also be re-used in a separate
GitHub Actions workflow that will handle the full tutorials-included
documentation build and deployment in the future.

The notebook-convertion step is moved into Python space, using
`nbconvert` as a library in order to parallelise the build process for
the tutorials, and to allow CI and developers calling `tox` directly to
specify the output directories for the built tutorials.

This reorganises the tutorial "conversion" utility to make it clearer
that what it's actually doing is just executing the tutorials.  The
script itself is changed to default to editing the files inplace, while
the `tox` job is updated to write the files into a special directory,
making it easier to clean up a dirty build directory and making it so
subsequent local executions will not pick up the converted files.

There was a worry that not being able to configure these would make it
more unpleasant to use `tox` for the jobs locally.