Add guide-toc-level option to customize guide TOCs

The new `--guide-toc-level` option, which defaults to 2, controls what
heading levels will appear in guides' tables of contents. Specifying
`--guide-toc-level=N` will result in headings at or above <hN> being
included in all guides' TOCs. Choosing a `guide-toc-level` of 1 disables
the TOC entirely.