Add one or more sub levels to TOC

[gautierchomel]

Finer navigation is necessary to a better reading experience.

For that, the epub generator should grab h2 and h3 tags to populate nested lists.

thorium-reader-doc/epub-doc/en/src/_includes/toc.njk

Line 20 in 9b0f566

<li><a href="{{EPUBRoot}}{{page.url}}index.xhtml">{{page.data.title}}</a></li>

[gautierchomel]

Handmade solution available.
See marisademeglio/eleventypub#6 (comment)