doc: decrease prominence of GOROOT_BOOTSTRAP

Go build scripts on UNIX (make.bash, all.bash) have not required
GOROOT_BOOTSTRAP since August 2017 (CL 57753). Windows build scripts
have followed suit since CL 96455. Most people building Go will have
a Go toolchain in their PATH and will not need to specify a different
toolchain.

This CL removes the GOROOT_BOOTSTRAP mention from the contribution guide
(it was there for Windows only, but it's not required anymore). The guide
is meant to be light and clear for beginners and is not supposed to be
a reference, so there's not need to keep mentioning GOROOT_BOOTSTRAP.

Also update install-source.html to reflect the current status quo,
where using the PATH is probably the first and most used default, and
GOROOT_BOOTSTRAP is just an option.

Change-Id: Iab453e61b0c749c256aaaf81ea9b2ae58822cb89
Reviewed-on: https://go-review.googlesource.com/c/go/+/224717


Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>