[style] Sanitise lectures for figsize and adjust config via myst-nb

[mmcky]

We need to review all lectures in lectures/*.md and:

• remove all use of locally adjusting figsize with plt.rcParams["figure.figsize"] = (11, 5)
• review size of matplotlib generated figures in lectures that have removed figsize removed -- if copilot can't generate the figures and compare, then a checkbox list of lectures in the PR (with line number references) that have figsize removed would be helpful for manual assessments.