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

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 #508, wait to avoid merge conflict)
• ar1_processes.md (added in #538 )
• business_cycle.md (checked by #538 , no need to add example admonition)
• cagan_adaptive.md (checked by #538 , no need to add example admonition)
• cagan_ree.md (checked by #538 , no need to add example admonition)
• cobweb.md (checked by #538 , no need to add example admonition)
• commod_price.md (checked by #538 , no need to add example admonition)
• complex_and_trig.md (added in #538 )
• cons_smooth.md (checked by #538 , no need to add example admonition)
• eigen_I.md (add one example admonition by #538 )
• eigen_II.md (add two example admonitions by #538 )
• equalizing_difference.md (exist current PR #522 , wait to avoid merge conflict)
• french_rev.md (exist current PR #525 , wait to avoid merge conflict)
• geom_series.md (exist current PR #524 , wait to avoid merge conflict)
• greek_square.md (exist current PR #525 , wait to avoid merge conflict)
• heavy_tails.md (add two example admonitions by #538 )
• inequality.md (add one example admonition by #538 )
• inflation_hisotry.md (exist current PR #525 , wait to avoid merge conflict)
• input_output.md (add two example admonitions by #538 )
• intro_supply_demand.md (add two example admonitions by #543 )
• laffer_adaptive.md (checked by #545 , no need to add example admonition)
• lake_model.md (checked by #545 , no need to add example admonition)
• linear_equations.md (added by #528 )
• lln_clt.md (added two examples by #536 )
• long_run_growth.md (checked, no example to add)
• lp_intro.md (checked by #531 )
• markov_chain_I.md (exist current PR #479 , wait to avoid merge conflict)
• markov_chain_II.md (add three example admonitions by #546 )
• mle.md (add one example admonitions by #546 )
• money_inflation.md (checked by #546 , no need to add example admonition)
• money_inflation_nonlinear.md (checked by #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 #534 , wait to avoid merge conflict)
• unpleasent.md (checked and no example added #535 )