docs: format with markdownfmt

[dvyukov]

Consider auto-formatting docs/*.md with markdownfmt as part of make presubmit:

go get -u github.com/shurcooL/markdownfmt
markdownfmt -w docs

#994 is the current diff it creates. It seems to break syzbot.md, but it all should be fixable.
Need to figure out how to vendor it.

[dvyukov]

Note: github.com/shurcooL/markdownfmt does not support reflowing to a given line width. There is an open issue for that:
shurcooL/markdownfmt#36