Fix docs.rs TOC if headings contain inline code

Example: https://docs.rs/tokio/latest/tokio/sync/index.html

Headings that contain inline code have more than two child nodes since each inline code and text between them is a separate child, so only taking the node value from the second child does not provide the full heading. Worse, if the heading starts with a code tag, the heading will currently be "null" (since nodeValue only works on text nodes).

header.innerText instead gets the complete rendered text of the whole heading across all children.

huhu / rust-search-extension

Fix docs.rs TOC if headings contain inline code #278