Makes the TOC at the top of the page collapsible.
Adds an explanation of the markup used to produce a collapsible TOC.
Adds a comment about the possibility of using an unordered list for the TOC,
and about using TOC only once per page.
Using `index.md` as name for the parent page within a section allows lazy developers
to skip defining `permalink`. This is already working and this PR just updates the documentation.