[Docs]: Add note on how to disable file downloads

## What is your suggestion?

Document `--disable-file-downloads` flag

## How will this improve the docs?

So people can find it instead of asking like this: https://github.com/coder/code-server/discussions/5886#discussioncomment-4406903

## Are you interested in submitting a PR for this?

If no one else does, yes.
