Adds MD010 (Hard tabs), MD012 (blank lines), MD029 (ordered
list prefix), MD030 (spaces after list markers), and fixes
remaining docs that were failing these tests
MD002 - First header should be level 1
MD006 - Start bullets at beginning of line
MD019 - No multiple spaces after header style
MD022 - Headers surrounded by blank lines
MD025 - Only 1 level 1 header
MD028 - No blank lines within blockquote
MD038 - Spaces inside code span elements
Add commented lines explaining the file, and including
rules that should pass already, and commented out rules
that can be added in the future, for reference.
The configuration is currently only specified in CI, by moving it into
`.mdlrc` we get immediate feedback locally in supported editors.
To ensure `mdl` is available it's also added to the `Gemfile`, though
CI will still use the version installed in the `gitlab-docs-lint` image.
