This commit introduces the following targets:

- `format-markdown`: automatically formats any Markdown files
- `check-markdown`: checks the formatting on Markdown files
- `lint-markdown`: lints Markdown files

These are executed implicitly as part of the `format`, `check` and
`lint` targets.

Markdownlint can be found at: https://github.com/markdownlint/markdownlint



Change-Id: I15740f2a7f949dc98780558270ea223b64c52c2f
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>