Rollup merge of #146170 - kumarUjjawal:master, r=Mark-Simulacrum

fix: offline rustdoc html missing favicon

As discussed in the rust-lang/rust#146149 the doc was missing the favicon icon when build locally and viewed on a browser. I changed the relative path and also now we explicitly copy both SVG and PNG.

<img width="1132" height="425" alt="Screenshot 2025-09-03 at 11 57 46 PM" src="https://github.com/user-attachments/assets/062cbb08-04ec-4d88-a43a-710fb6190f82" />

Author: matthiaskrgr