Cleaner markdown output for API docs

[dsyme]

The markdown output for API docs is messy - there's lots of embedded HTML and I'm not sure it renders properly

This branch has the start of an attempt to generate less HTML: https://github.com/dsyme/FSharp.Formatting/tree/apimd However it's still got many problems. After running tests the typical markdown output can be viewd at, say

FSharp.Formatting\tests\FSharp.ApiDocs.Tests\output\md\FsLib2_examples\reference\fslib-commentexamples.md