Update src/solutions/7_Tight-binding_model_sol.md, src/7_tight-binding_model_sol.md, mkdocs.yml files

Add prior knowledge to einstein lecture

See merge request !57

Update src/solutions/1_einstein_model.md, mkdocs.yml files

See merge request !51

Random walk plot

See merge request !56

try to fix requirements

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Update symlinks

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Fix plotly plots

Closes #61

See merge request !48