Add `prf:example` environment to Examples in the lectures

[longye-tian]

We want to add

```{prf:example}
:label: example_label

(We need to have labels if there are more than two examples and there is a space after the :, and there is no space for prf:example).

to examples to the intro lecture series.

• ak2.md (exist current PR [ak2] Update to Transitions in an Overlapping Generations Model lecture #508, wait to avoid merge conflict)
• ar1_processes.md (added in [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• business_cycle.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• cagan_adaptive.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• cagan_ree.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• cobweb.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• commod_price.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• complex_and_trig.md (added in [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• cons_smooth.md (checked by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 , no need to add example admonition)
• eigen_I.md (add one example admonition by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• eigen_II.md (add two example admonitions by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• equalizing_difference.md (exist current PR [equalizing_difference] Add an exercise #522 , wait to avoid merge conflict)
• french_rev.md (exist current PR [french_revolution] [greek_square] [inflation_history] Tom's edits for July #525 , wait to avoid merge conflict)
• geom_series.md (exist current PR [geom_series] Add an exercise #524 , wait to avoid merge conflict)
• greek_square.md (exist current PR [french_revolution] [greek_square] [inflation_history] Tom's edits for July #525 , wait to avoid merge conflict)
• heavy_tails.md (add two example admonitions by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• inequality.md (add one example admonition by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• inflation_hisotry.md (exist current PR [french_revolution] [greek_square] [inflation_history] Tom's edits for July #525 , wait to avoid merge conflict)
• input_output.md (add two example admonitions by [ar1_processes] to [eigen_II] [heavy_tails][inequality]Example and spelling #538 )
• intro_supply_demand.md (add two example admonitions by [intro_supply_demand] Update spellings and example admonitions #543 )
• laffer_adaptive.md (checked by [laffer_adaptive] [lake_model] spelling and check example admonition #545 , no need to add example admonition)
• lake_model.md (checked by [laffer_adaptive] [lake_model] spelling and check example admonition #545 , no need to add example admonition)
• linear_equations.md (added by [linear_equations]Update the example environment #528 )
• lln_clt.md (added two examples by [lln_clt] spell check and example admonition #536 )
• long_run_growth.md (checked, no example to add)
• lp_intro.md (checked by [lp_intro]Update lp #531 )
• markov_chain_I.md (exist current PR Update made to Markov Chains: Basic Concepts lecture #479 , wait to avoid merge conflict)
• markov_chain_II.md (add three example admonitions by [markov_chains_II] to [money_inflation]spelling and example admontion #546 )
• mle.md (add one example admonitions by [markov_chains_II] to [money_inflation]spelling and example admontion #546 )
• money_inflation.md (checked by [markov_chains_II] to [money_inflation]spelling and example admontion #546 , no need to add example admonition)
• money_inflation_nonlinear.md (checked by [markov_chains_II] to [money_inflation]spelling and example admontion #546 , no need to add example admonition)
• monte_carlo.md
• networks.md
• olg.md
• prob_dist.md
• pv.md
• scalar_dynam.md
• schelling.md
• short_path.md
• simple_linear_regression.md
• solow.md
• supply_demand_heterogeneity.md
• supply_demand_multiple_goods.md
• time_series_with_matrices.md (exist current PR [time_series_with_matrices] Address feedbacks #534 , wait to avoid merge conflict)
• unpleasent.md (checked and no example added [unpleasent] spell check #535 )

[mmcky]

thanks @longye-tian this would be a great task. When making edits it would be helpful to edit in batches to reduce the possibility of merge conflicts.

[longye-tian]

thanks @longye-tian this would be a great task. When making edits it would be helpful to edit in batches to reduce the possibility of merge conflicts.

Hi Matt,

I definitely agree. I will first choose the lecture without any open pull request to work on.

Best,
Longye