Implement artifact-level render command with Markdown output and update docs

examples/render/output-markdown.md

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+# Transcript
+
+[00:00:01–00:00:04] **Eric Rakestraw:** Hello there.
+
+[00:00:05–00:00:08] **Mike Brown:** Welcome back, everyone.
+
+[00:00:09–00:00:10] **Eric Rakestraw:** *Yeah.*