Fix the code blocks in the documentation

[adithyaov]

Use the markdown-doctest on the new-rel branch here: composewell/markdown-doctest#6
You can use the following to run it on all the markdown files:

fd . -e md -0 | while IFS= read -r -d '' file; do echo "CMD: markdown-doctest $file" && markdown-doctest "$file" || break; done