Expand fathead documentation

I changed the big code blocks into some prettier lists and added an intro section.

I may have missed some details, but I think it's better than it was before.
Made some changes:

Fatheads are database backed, key-value instant answers. The keys of the database are typically words or phrases,  and they are used as the triggers for the instant answer. When a database key is queried, the corresponding row from the database is returned, which is typically a paragraph of text. In order to develop an instant answer, a program must be written which generates an "output.txt" file which indicates the keys and values for the database. The program may be written in any reasonable language, and if necessary, will be run periodically to keep the database current.

The "output.txt" file that is generated will be consumed by the DuckDuckGo backend, cleaned up (details below) and then finally entered into an SQL database.