fix: correct pipe precedence docs — |> binds tighter than +

R's |> has higher precedence than +, so parentheses ARE required
around the ggplot chain before piping to civilytics_logo():

  (ggplot(df, aes(x, y)) + geom_point() + theme_civilytics()) |>
    civilytics_logo()

Updated docs, examples, and pipe test accordingly.

.Rbuildignore

@@ -9,3 +9,4 @@
 ^\.claude$
 ^\.playwright-mcp$
 ^civilytics-site\.png$
+^Rplots\.pdf$