Add support for markdown output format in crystal docs command

[kojix2]

Hello,

This pull request adds Markdown documentation generation to the crystal docs command, utilizing Vibe Coding techniques for the Crystal language.

crystal docs --format markdown

Background

Recently, Vibe Coding (AI-assisted coding) has become increasingly popular. However, AI is not always familiar with the details of Crystal's libraries and APIs. Therefore, when explaining a library's API to an AI, a documentation format that is easy for both humans and AI to read is essential.
Markdown is the optimal format for such communication.

Development Process

This feature was developed through the following process:

1. Investigated how to add Markdown output to crystal docs using DeepUncyclo.com
2. Created an implementation plan on DeepUncyclo.com
3. Reviewed and refined the plan on DeepUncyclo.com to make it simpler and more robust
4. Reviewed the proposal with VSCode/Cline (ChatGPT 4o)
5. Implemented the feature with VSCode/Cline (Sonnet-3.7)
6. Built, tested, and fixed issues with VSCode/Cline (Sonnet-3.7)

Summary

This pull request demonstrates which files need to be modified within the relatively large Crystal project to enable this functionality. I hope the Crystal team will consider an official implementation.

[straight-shoota]

Please open an issue first to further discuss the feature independent of its implementation (c. f. https://github.com/crystal-lang/crystal/blob/master/CONTRIBUTING.md#using-the-issue-tracker). Thanks 🙇