doc: remove all docs not tied to distribution

They have moved to x/website in CL 291693.
The docs that are left are the ones that are edited at the same time
as development in this repository and are tied to the specific version
of Go being developed. Those are:

 - the language spec
 - the memory model
 - the assembler manual
 - the current release's release notes

Change-Id: I437c4d33ada1b1716b1919c3c939c2cacf407e83
Reviewed-on: https://go-review.googlesource.com/c/go/+/291711


Trust: Russ Cox <rsc@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>