Add a summary report set from Markdown

from-markdown takes a hand-written summary such as SUMMARY.md and sets it
in the full report's dress: the same cover, header, footers, table of
contents, headings and grids. The shared macros move to macros.html.j2 so
both templates draw from them, and layout.render_pdf takes the template and
document title to use.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -40,6 +40,8 @@ dependencies = [
     # needs the native Pango and cairo libraries, which pip does not install -- see the README.
     "jinja2>=3.1.6",
     "weasyprint>=69.0",
+    # The summary report, which is written in Markdown and set in the same template.
+    "markdown-it-py>=4.0.0",
 ]
 
 [project.optional-dependencies]