Get rid of Markdown writer

This crate contains an ad-hoc, incomplete, and probably also bug-ridden implementation of a Markdown _writer_: https://github.com/killercup/rust-docstrings/blob/aa705431de73da8bd59c4168148aedb77e020dd9/src/to_md.rs

I'd be great to upstream/outsource that.

Start by reading src/to_md.rs, the linked implementation, and the crate context around its Markdown writer. Investigate whether the writer can be upstreamed or replaced by an external implementation; done means the ad-hoc writer is no longer maintained in this crate and the chosen replacement is integrated.

索引モデルが issue の本文から書いたものです。