feat: add Markdown rendering package

[rsubrama83]

Closes #33

What

New package ShinyPDF.Markdown with a Markdown() extension on IContainer (namespace ShinyPDF.Fluent):

page.Content().Markdown(markdown, options => options.BaseFontSize = 11);

Markdown is parsed with Markdig and composed from existing elements (Column, Row, Text, Border, Line), so it wraps and pages like any other content.

Supported: headings, paragraphs, hard line breaks, bold, italic, strikethrough, inline code, code blocks, links and autolinks, nested bullet and numbered lists, blockquotes, horizontal rules. Raw HTML is rendered as plain text (never interpreted), images as their alt text. Tables and task lists are left for later.

MarkdownOptions covers base font size, spacing, list indent, link/code/quote/rule colors and a heading style override.

Why a separate package

The core ShinyPDF package keeps zero new dependencies; only consumers who want Markdown pull in Markdig.

Release impact

• feat: commit: merging releases 2.1.0.
• publish.yaml now builds and packs both projects with the same version. ShinyPDF.Markdown depends on ShinyPDF at that version and on Markdig 1.4.0.
• First publish of the new package ID ShinyPDF.Markdown: please check the NuGet trusted publishing setup allows it.

Verification

• 168 unit tests pass in Release (18 new in MarkdownTests, asserting on the composed element tree and styles, plus a full PDF generation).
• Example render (MarkdownExamples) checked visually, image added to the docs.
• Local dotnet pack inspected: nuspec dependencies, icon, readme and license are correct.

Docs

• New how-to: docs/how-to/render-markdown.md
• README section, docs index link, AGENTS.md updated (layout, pack command, current focus).

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