Documentation: Add mdl style file

Documentation: Add mdl style file

This adds a style file for the markdown linter mdl.

The tool can be found on archive.org at the URL:
https://web.archive.org/web/20220407032312/https://github.com/markdownlint/markdownlint

This does 2 things:
- Sets that line length limit to 72 characters as requested in the docs
about writing the documentation.

- Excludes several rules that were added for a particular markdown
parser. My opinion is that these rules make the text versions of the
markdown harder to read.

To use this style file, run:
$ mdl -s Documentation/.mdl_style.rb

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I98289492ae3e920d440f0e5c308a3590fb89d9fd
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63805
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <patrick@coreboot.org>