make ToC optional

[therealglazou]

edasfr thinks ToC should be optional

Source: https://n.survol.fr/n/e0-le-retour-du-fils-de-la-vengeance

[frivoal]

Inline in the doc, you also noted:

He also wants the spine to be dropped. So how do we define the first document to read?

The first document to read is simply index.html (see also #13). If it has a rel=next, the next document after that is what it points to. If there is neither a rel=next nor a toc/spine, then there is no "next" document after the index.html and any other page needs to be navigated to using hyperlinks.