update admonitions for migrated lectures

Author: HumphreyYang

lectures/geom_series.md

@@ -29,6 +29,12 @@ kernelspec:
 :depth: 2
 ```
 
+```{admonition} Migrated lecture
+:class: warning
+
+This lecture has moved from our [Intermediate Quantitative Economics with Python](https://python.quantecon.org/intro.html) lecture series and is now a part of [A First Course in Quantitative Economics](https://intro.quantecon.org/intro.html).
+```
+
 ## Overview
 
 The lecture describes important ideas in economics that use the mathematics of geometric series.

lectures/lp_intro.md

@@ -12,6 +12,12 @@ kernelspec:
 (lp_intro)=
 # Linear Programming
 
+```{admonition} Migrated lecture
+:class: warning
+
+This lecture has moved from our [Intermediate Quantitative Economics with Python](https://python.quantecon.org/intro.html) lecture series and is now a part of [A First Course in Quantitative Economics](https://intro.quantecon.org/intro.html).
+```
+
 In this lecture, we will need the following library. Install [ortools](https://developers.google.com/optimization) using `pip`.
 
 ```{code-cell} ipython3

lectures/short_path.md

@@ -27,6 +27,12 @@ kernelspec:
 :depth: 2
 ```
 
+```{admonition} Migrated lecture
+:class: warning
+
+This lecture has moved from our [Intermediate Quantitative Economics with Python](https://python.quantecon.org/intro.html) lecture series and is now a part of [A First Course in Quantitative Economics](https://intro.quantecon.org/intro.html).
+```
+
 ## Overview
 
 The shortest path problem is a [classic problem](https://en.wikipedia.org/wiki/Shortest_path) in mathematics and computer science with applications in