Fix broken markdown and other lint problems

What does this MR do?

Fixes some issues with the style guide introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22249. No longer trying to nest fenced code blocks.

Also fixes some things picked up by markdownlint and proselint.

Related issues

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22249

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Technical writer's review (required)