Docs refresh

[tomchristie]

• Push back on "performance". A bit silly really. Was important at one point in order to demonstrate that the ASGI approach was not any worse than tightly integrated alternatives. But we've established that point now.
• Drop "Support for HTTP/2 is planned".
• Add "Why ASGI" section to better provide the rationale for the ASGI ecosystem.
• Include "Alternative ASGI servers" on the README (as well as the existing section on the homepage.)
• Drop "Designed & built in Brighton, England." - I think there's some validity to design approaches that have a strong initial author voice, because it helps establish a clear direction and design intent, but it's also diminishing to the vast number of folks who've worked on it over the years, and the multitude of contributions of all different kinds that make it possible.

[aminalaee]

There's an extra file added docs/.DS_Store.

[florimondmanca]

Makes complete sense. 👍

[tomchristie]

Thanks folks! Comments addressed.
I've also bumped up encode/starlette#1477 which is similar.