Document the mdpdf command in the source-install guides

The distribution shipped sks/tns/ and the mdpdf command in the previous
snapshot without either install guide mentioning them, so the only way to
discover the command was to read env/runtime.env.  Both source-install
guides now describe it: what it is for (Markdown that is not Klammertext,
rendered through a headless browser), that runtime.env defines it -- so a
shell started before the installation does not have it -- the two things the
distribution does not install (the renderer's virtual environment, created
by "md_to_pdf.py --setup", and a Chromium-based browser), the MDPDF_*
variables that change its defaults, and where the stylesheet is.

 (from dev d8ea0e973914)

doc/install/macos_source_install.md

@@ -145,7 +145,58 @@ xelatex --version
 ktext hello.kt -t pdf         # writes hello.pdf
 ```
 
-## 6. Updating
+## 6. Optional: the `mdpdf` command (Markdown to PDF)
+
+Klammertext ships a second, independent route in `sks/tns/`: `mdpdf` renders
+a **Markdown** file to PDF through a headless browser, bypassing Klammertext
+entirely. It is for a document that is not written in Klammertext — one you
+have not converted yet, or one whose Markdown is not worth converting.
+
+The command is a shell function that `env/runtime.env` defines, so if you
+source that file (section 3) you already have it — **in shells started after
+the installation**. In a shell that was already running, source it again:
+
+```sh
+source "$KLAMMERTEXT_HOME/env/runtime.env"
+type mdpdf                    # mdpdf is a shell function
+```
+
+Two things it needs that the distribution does not install:
+
+```sh
+# 1. The Markdown renderer, in a virtual environment of its own.  PEP 668
+#    forbids installing it into the system Python, so this is not optional
+#    and not a system package.
+python3 "$KLAMMERTEXT_HOME/sks/tns/md_to_pdf.py" --setup
+
+# 2. A Chromium-based browser -- Brave, Chrome or Chromium -- which is the
+#    renderer.  Any one of them will do, and an .app in /Applications is
+#    found without being told where it is.
+brew install --cask chromium
+```
+
+Then:
+
+```sh
+mdpdf notes.md                # writes notes.pdf beside it
+mdpdf notes                   # the .md may be left off
+```
+
+It supplies the fonts, the size matching and the code wrapping, and completes
+on `*.md` at the TAB key. Each default is a variable you may set in
+`~/.zprofile` — `MDPDF_SERIF`, `MDPDF_SANS`, `MDPDF_MONO`, `MDPDF_MATCH`,
+`MDPDF_WRAP`, and `MDPDF_BROWSER` for a browser installed somewhere the
+command does not look. Any option of the underlying `md_to_pdf.py` may also
+be given on the command line, where it overrides the default:
+
+```sh
+mdpdf notes.md --paper letter --margin 0.75
+```
+
+The stylesheet it applies is `sks/tns/markdown.css`; copy it, edit the copy,
+and pass `--css yourcopy.css` to render to your own taste.
+
+## 7. Updating
 
 To update an existing source installation to the latest version: