docs: fix MD031 markdown lint

[xtqqczze]

MD031/blanks-around-fences : Fenced code blocks should be surrounded by blank lines

[PhilipMetzger]

if the project adopts a MD linter we first should find consensus on that before starting to apply it.

[martinvonz]

I agree. But markdownlint seems to not be a formatters but a linter. We can adopt both a formatter and markdownlint, I suppose. markdownlint presumably doesn't make suggestions that would go against any particular formatter.

[steveklabnik]

As a random aside, I've always written Markdown in this way, I suspect a lot of implementations these days would render without the spaces, but in the earlier days, many didn't.

I have no opinions on markdown linters or formatters, I haven't really used any myself.