feat: Add rawMarkdown function to Page

[andrewtdiz]

This commit introduces a new rawMarkdown function, allowing a page's content to be rendered back to its original Markdown format.

Useful for copyMarkdown button in static websites or rendering as plain Markdown

This commit:

• Adds a rawMarkdown struct to the Page
• Implements the Markdown rendering logic in a new src/render/markdown.zig file.
• Updating simple test layout and snapshots to include and verify the raw Markdown output.

The <template> tag is preferred for Copy Markdown functionality as it:

• Does not render to the DOM
• Is ignored by search engine SEO.
• Is valid, semantic HTML5.