MAINT: Refactor graphviz figures to be lecture_specific figures.ipynb assets

[mmcky]

This PR migrates the relevant sections of _static/lecture_specific/graphviz to be contained in figures.ipynb notebooks in

• markov_chains_I
• markov_chains_II
• networks
• lake_model

so the figures are directly associated with the lectures in which they appear.

This commits both the png asset and the text tikz output.

Fixes #422

• [x] merge after https://github.com/QuantEcon/lecture-python-intro/pull/423

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[mmcky]

@jstac this is ready for your final review and merge.

[jstac]

Many thanks @mmcky , much appreciated. Please merge when ready.

[mmcky]

• [x] checked paths in the lectures for graphviz and doesn't appear to be referenced. Merging this now.