Fix markdown linter errors

v0.13 of the `markdownlint` gem just got released and it has
a few new rules which are docs ended up failing. This fixes
those linter errors so that CI is no longer red.

Rules:
MD055 - Tables: Each row must start and end with a '|'
MD057 - Tables: In the second row every column must have at least '---',
        possibly surrounded with alignment ':' chars