doc: lint Markdown for trailing spaces and code blocks

doc: lint Markdown for trailing spaces and code blocks

MD009 Trailing spaces.
MD031 Fenced code blocks should be surrounded by blank lines.

https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md

Change-Id: I6a2fff4afa8224442d90b30f84444b4a888fcef2
GitHub-Last-Rev: 67cf146b8501096949faf0155239770d4f6a73b4
GitHub-Pull-Request: golang/go#44285
Reviewed-on: https://go-review.googlesource.com/c/go/+/292409
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alex Rakoczy <alex@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Alex Rakoczy <alex@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>