doc: remove docs.html

This page has moved to x/website in https://golang.org/cl/197638.

Updates #33637
Updates #29206

Change-Id: I4f5f7822a2bf540a3911470548d38ccc7a66b74c
Reviewed-on: https://go-review.googlesource.com/c/go/+/199117
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>