Do not include headers in search index

[notriddle]

This significantly shrinks the pre-compressed search index:

$ du -h searchindex-old.js searchindex-new.js
26M	searchindex-old.js
19M	searchindex-new.js

And shrinks the search index even after it's gzipped:

$ du -h searchindex-old.js.gz searchindex-new.js.gz
4.5M	searchindex-old.js.gz
3.3M	searchindex-new.js.gz

This change requires a newer version of mdBook, with rust-lang/mdBook#1637

Screenshots

Before:

After:

[ehuss]

Thanks! This seems reasonable to me. Since there is such a large number of pages, I think having the search target individual sections probably isn't super useful.