Docs: Update serving docs instructions

[Samoed]

I was trying to debug #12527 and follow instructions from https://github.com/gradio-app/gradio/blob/main/CONTRIBUTING.md#%EF%B8%8F-gradio-website--docs

pnpm i 
pnpm dev

But when I tried to access served docs I got strange window on newly install chromium

And there is nothing on port 5173