Table of contents

[bjones1]

Ideas for a table of contents:

1. Current approach: allow any Markdown and simply put that in the TOC.
2. Auto-TOC: a tag that contains a list of files, one per line. These specify the TOC PreTeXt-style.
3. Filesystem: a tag that dumps all files in the project as the TOC, sorted by name.

These can easily coexist in the toc.md file.