`deno doc`: add Native Markdown Export Functionality

[Aziz-TheDarkCode]

Currently, the deno doc command provides documentation generation capabilities, but lacks a direct option to export documentation in Markdown (.md) format. This feature request proposes adding a --markdown or --md flag to enable seamless documentation export.

Add a new CLI flag to deno doc that allows exporting documentation as a Markdown file:

# Proposed usage
deno doc --markdown web/ui/button.tsx
deno doc --md web/ui/button.tsx

[lucacasonato]

We already support HTML, JSON, and the CLI text output. Why do you need markdown instead of using the HTML output?