Add markdownlint config file, address some issues

Mostly related to whitespace (newlines / trailing),
lots of issues remaining to fix in the docs directory.

As usual linting takes most effort when you start,
but from there it is easier to keep code and docs clean.

Author: afbjorklund

.markdownlint.json

@@ -0,0 +1,6 @@
+{
+  "no-inline-html": false,
+  "no-trailing-punctuation": false,
+  "ul-style": false,
+  "line_length": false
+}