feat(p/demo/ui/md): add markdown generation framework

[trasnake87]

Summary

This PR introduces p/demo/ui/md, a markdown generation framework for Gno.

Submission for bounty #2753

Features

• Builder pattern with method chaining
• Standalone functions for composition
• Full markdown support
• 80%+ test coverage

Demo

Updated r/demo/userbook and r/demo/microblog.

Resolves #2753

[Gno2D2]

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[notJoon]

Note that concatenating strings with + is way more cheaper than using string.Builder in gno