Do not generate unused parts of toctree

Do not generate unused parts of toctree

Our css only displays up to depth 3 of the toctree, partially
because the API reference content explodes at depth 4 and that would
not be pretty to see in the sidebar.  However, we would previously
always generate HTML for the full toctree and hide parts with CSS.
For the apiref, this proved to be about 65k per html file, and we
have one html file per function/type/macro.

Limit the depth of the toctree that gets generated to save on space
in the release tarball.

Unfortunately, there seems to be a Sphinx bug wherein the toctree
will only be generated to depth 1 for a document at a depth greater
than the maxdepth of the toctree, so the sidebar table of contents
on individual apiref pages will just be the toplevel toctree.
This issue is being tracked at
https://bitbucket.org/birkenfeld/sphinx/issue/1046/

ticket: 7466 (new)
tags: pullup
target_version: 1.11