Small Markdown-to-Guppy component.

GPUI 0.2.2 does not expose a standalone markdown viewer in the dependency surface Guppy uses, so this component parses Markdown with Erlang/OTP's :shell_docs_markdown and renders a deliberately small subset into ordinary Guppy IR. Supported today: headings, paragraphs, unordered/ordered lists, and inline bold/italic/code/link-ish runs.