Add `markdownlint` linter and fix issues. Config is based on the one from electron's repo with a few rules relaxed.
* First pass Signed-off-by: jolheiser <john.olheiser@gmail.com> * More changes Signed-off-by: jolheiser <john.olheiser@gmail.com> * Redirects Signed-off-by: jolheiser <john.olheiser@gmail.com>