[markov_II] FIX: fix state transition figure in lecture

[mmcky]

This fixes #394

• [x] adds source code for newly generated example4.png file
• [x] check scale in preview

There is some discussion around making these figures using mermaid as the source is then included in the lecture and moves us closer to self contained documents. However in the short/medium term we will need to assess LaTeX compatibility.

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[mmcky]

@jstac here is the preview

https://661ddef5826bb170f51dd970--taupe-gaufre-c4e660.netlify.app/markov_chains_ii#example-a-periodic-chain

[jstac]

Thanks @mmcky

@HumphreyYang , could you please do a first round review?

[mmcky]

Closing this PR and will open another based on comment in #394