Better documentation for creating training configs

[AkshitaB]

• The guide gives an example of a training config, but not an exhaustive list of what fields are possible to include.
• The Vocabulary and Trainer fields are general, while for the model and dataset reader, the fields will depend on specific classes.
• It will be useful to have this documented for someone looking to get started with AllenNLP.

Create a dedicated page in docs or add link here .

[gabeorlanski]

Hi, I would be interested in contributing to this as trying to understand configs definitely was a pain point when trying to learn AllenNLP. What repo should I add the docs to?