Adding section titles to ToC / sidebar

[rimutaka]

@ehuss , Eric, would you consider a PR for the following feature ...

At the moment the sidebar is constructed out of the items listed in SUMMARY.md. It would be helpful to include an additional level out of Hx titles found in the markdown content in the ToC as an optional feature.

At the moment Hx tags are wrapped into with #-anchors, so we just need to add them to the ToC in the right place.
The list of H-levels can be included in book.toml as a setting.