Add scorch index options to FTS docs for 5.5

New index engine is  "developer preview" only for 5.5

1. New feature adds a drop-down box to the index creation UI. (Available since 5.5 Beta)
2. And modifies the JSON used to define an index.
3. It provides reduced index size (on disk).
4. It indexes and processes mutations faster.

I will provide more details as-needed, but for now keeping this ticket as a placeholder.