slides: fix closing slide — centered content clobbered the reference footer

The .center class vertically centers the slide body, which overlapped the
bottom-pinned ::: aside (the Boyd & Vandenberghe line). Drop .center and move
both references into a normal-flow Resources list.

Author: bnaras

slides/intro.qmd

@@ -207,16 +207,18 @@ Disciplined nonlinear programming: [cvxr.rbind.io/examples/dnlp/tutorial](https:
 Everything expanded, with more examples: [cvxr.rbind.io](https://cvxr.rbind.io)
 :::
 
-## Let's begin {.center}
+## Let's begin
 
 Open the book at **A Gentle Introduction** and run the first chunk.
 
 <br>
 
-*Convex Optimization in R Using CVXR* · Fu, Narasimhan & Boyd
+**Resources**
 
-[cvxr.rbind.io](https://cvxr.rbind.io) · Fu, Narasimhan & Boyd (2020), *JSS* 94(14)
+- The book & all examples: [cvxr.rbind.io](https://cvxr.rbind.io)
+- The package paper: Fu, Narasimhan & Boyd (2020), *JSS* **94**(14)
+- The standard reference: Boyd & Vandenberghe, *Convex Optimization* (2004)
 
-::: aside
-The standard reference: Boyd & Vandenberghe, *Convex Optimization* (2004).
-:::
+<br>
+
+*Convex Optimization in R Using CVXR* · Fu, Narasimhan & Boyd