Document font used in benchmark charts (astral-sh#1309)

The labels were missing when I generated the charts on my Arch machine. 

Inspecting the intermediate SVG showed that we use Google's Roboto font
(and the rendering library doesn't support fallback fonts).

I installed the font and TADA, the labels appeared. I extended our
documentation to mention the required fonts.

BENCHMARKS.md

@@ -97,6 +97,8 @@ cargo run -p puffin-dev render-benchmarks install-warm.json --title "Warm Instal
 cargo run -p puffin-dev render-benchmarks install-cold.json --title "Cold Installation"
 ```
 
+You need to install the [Roboto Font](https://fonts.google.com/specimen/Roboto) if the labels are missing in the generated graph.
+
 ## Acknowledgements
 
 The inclusion of this `BENCHMARKS.md` file was inspired by the excellent benchmarking documentation