No `quiet` arg for `build_site`

[givison]

I'm working on debugging an issue running build_site where a function is hanging but doesn't error. I was able to narrow it down to one of my pages and made further progress on debugging by running build_article with quiet = FALSE specifically for that page. But this would have been much more straightforward if build_site had a quiet argument itself.

[hadley]

I think this would be a great opportunity for a community contribution. It's quite a bit of work, but I think the individual tests are reasonable simple:

• Add a quiet argument to build_site()
• Pass it through to build_site_external()/build_site_local()
• In build_site_local() pass it through to each build_ function that already has a quiet argument
• Add a snapshot test to make sure it actually works 😄
• Document the new quiet parameter — this is slightly tricky because it doesn't actually quiet all output (since we want to keep the existing headers), but I don't think it's worth fixing this problem in the code.