Solutions for missing dependencies

Docr is currently using [Markd](https://github.com/icyleaf/markd) to handle markdown parsing for doc content. This is not ideal as the API is poorly documented and [is not reusable](https://github.com/devnote-dev/docr/blob/1c8abfa34594f885d477d904be5fa466a411b702/src/renderer.cr#L101). On top of that, the API itself is difficult to work with; there are many bugs with the current formatting of doc content due to this. At present there is no markdown formatting applied to type summary text, this is likely the cause. It's safe to say this will not be sustainable for the long-term.

I have put effort into [Marker](https://github.com/devnote-dev/marker), an alternative token-based markdown parser, but it is still far from production ready. Until then there should be another alternative Docr can use before it can be released. As a last resort the HTML content may have to be stripped from type summary text and just grin-and-bear Markd until Marker is ready.

Markdown code blocks are currently tinted with a dark-ish brown (?) colour. Ideally this should be highlighted as Crystal code so I am exploring integrating [Tartrazine](https://github.com/ralsina/tartazine).

Start by reading src/renderer.cr around line 101 to understand how Markd is used for doc content and type summaries. Compare the stated alternatives, Marker and Tartrazine, and verify the chosen direction against the reported missing formatting and code-block highlighting; done means the project has a production-ready dependency path for those cases.

索引モデルが issue の本文から書いたものです。