More BTTF Docs updates

cc// @russellholt @jagtalon @jdorweiler @mwmiller 

- Added `exactMatch` and `Boost` to `normalize: {}` section [[link]](https://github.com/duckduckgo/duckduckgo-documentation/compare/zaahir;docs-additions?expand=1#diff-72e30d82d789ae1f596c214aa831346aR115)
- Added explanation of `sort_fields` and `sort_default` [[link]](https://github.com/duckduckgo/duckduckgo-documentation/compare/zaahir;docs-additions?expand=1#diff-72e30d82d789ae1f596c214aa831346aR316)
- Added docs for event handlers, `onItemSelected`, `onShow`, etc. [[link]](https://github.com/duckduckgo/duckduckgo-documentation/compare/zaahir;docs-additions?expand=1#diff-72e30d82d789ae1f596c214aa831346aR350)
- Switched to H3 tags for all fn names
- Using bullet lists for 2nd level properties
- Wrap input types with `` rather than ** (looks better)