doc/README: suggest a way to preview next content, tweak release steps

Smaller edits are usually fine to do without previewing, since Markdown
can be intuitive. But for larger changes including re-ordering sections
and such, it can be helpful to quickly see the end result. Write down a
way to do that.

Update the release steps to capture that the doc/next content will move
to x/website before RC 1, when the complete release note draft is ready.

For #64169.

Change-Id: Ie554ed5294ce819fd0689e2249e6013826f0c71f
Reviewed-on: https://go-review.googlesource.com/c/go/+/587922
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

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