Migrate AR(1) Lecture (#335)

* migrate ar1 * remove intro tag * add migration admonition * update admonition text' --------- Co-authored-by: mmcky <mamckay@gmail.com>

Author: HumphreyYang

lectures/_toc.yml

@@ -46,6 +46,7 @@ parts:
 - caption: Stochastic Dynamics
  numbered: true
  chapters:
+ - file: ar1_processes
  - file: markov_chains_I
  - file: markov_chains_II
  - file: time_series_with_matrices