docs and refactor

examples/demo.md

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+# Textual Markdown Browser
+
+Welcome fellow adventurer! If you ran `markdown.py` from the terminal you are viewing `demo.md` with Textual's built in Markdown widget.
+
+The widget supports much of the Markdown spec. THere is also an optional Table of Contents sidebar which you will see to your left.
+
+## Do You Want to Know More?
+
+See [example.md](./example.md) for more examples of what this can do.