Implement `dolt_help` set of system tables

[timsehn]

We should have a dolt_help system table with the following schema:

target (key): ie. dolt_checkout, dolt_diff, dolt_hashof, etc type: system table, procedure, function, variable short description: the short help text long description: long help text arguments: a json blob describing the arguments

This would help move our documentation in product and be generally useful. We could even generate the docs site documentation from this code.