Move details of markdownlint mostly to the documentation
index for a SSoT, and link apropriately

Those guidelines where based on
https://gitlab.com/gitlab-org/gitlab-docs/issues/311 which seems
to be fixed.

Marcel Amirault authored 5 years ago and Achilleas Pipinellis committed 5 years ago

Clean up issues that may cause the docs-lint
test to fail, such as trailing whitespace, no
EOF newline, blockquotes, etc

Clarify rules for code blocks, including spaces
above and below, and to use tilde when code blocks
contain triple backticks