All headings are shown in PDF TOC

[timobrembeck]

Describe the Bug

When PDFs are exported, all headings from <h1> to <h6> are listed in the table of contents.
Instead, we should only list <h1> which originated from real page titles.

Steps to Reproduce

1. Create page with several sub-headings
2. Export PDF

Expected Behavior

The table of contents should list all pages in the export - not all sub-headings in all pages.

Actual Behavior

All sub-headings are contained in the TOC as well

Additional Information

I'm not sure whether there is a pretty way to solve this - our library just takes all headings, see docs.
I see two workarounds for this:

1. Hide the additional levels in the TOC via CSS (preferred)
2. Convert all headings to generic <div>s during PDF export and style them the way headings normally look